Where did these aches and pains come from? Where did these aches and pains come from?

Some of it might be simple muscle soreness. People tend to be less active in middle age than they were in young adulthood, and if you’re not used to working a specific muscle group, tasks like raking the yard or shoveling snow can leave you achy after, said Scott Trappe, a professor of human bioenergetics and director of the Human Performance Laboratory at Ball State University.

Muscle mass also starts to naturally decline in middle age, which can result in painful joints. “What muscle does is it takes up some of the load that you’re carrying and frees the joints from the pain,” said Dr. Arun Karlamangla, a professor of medicine at the University of California, Los Angeles, who specializes in geriatrics.

In addition, our joints become stiffer as we age because of the accumulation of wear and tear, which can result in scar tissue. “The tendons and the muscles lose a little bit of their plasticity,” Dr. Trappe said. And when you combine weak muscles and stiff joints with movement — especially movements that are rapid, twisting or high force — you can get “something that tears or pops,” he said.

There is an added risk of injury in women, as their bones become weaker when estrogen drops during menopause. “You don’t feel bone density loss. It doesn’t hurt,” said Dr. Stephanie Faubion, the director of the Mayo Clinic’s Center for Women’s Health and the medical director of The Menopause Society. “You don’t feel anything about that until you break something.”

Is there anything I can do?

All of these issues — muscle strength, bone density and joint health — can be improved with exercise. Strength training is critical to offset declines in muscle mass and bone density, and aerobic exercise has important cardiovascular and other health benefits. “The silver bullet is physical activity,” Dr. Karlamangla said. That goes not only for muscle and joint issues, but virtually any age-related changes.

Dr. Karlamangla added that there’s no reason you shouldn’t continue to push yourself in your workouts, like running instead of walking or increasing the weights you’re lifting. “Even a little bit of physical activity is good,” he said, but to get the greatest benefit, aim for moderate or vigorous intensity. Talk to a personal trainer, physical therapist or your doctor if you have specific injuries or health concerns you need to take into account.

Why am I gaining weight all of a sudden? Why am I gaining weight all of a sudden?

For decades, the general assumption was that people struggle more with their weight in midlife because their metabolism suddenly slows down. But a 2021 paper published in the journal Science cast doubt on that conclusion. The research showed that the amount of calories people burn, both through their resting metabolism and their daily activities, is actually fairly stable from ages 20 to 60. (Metabolism is faster from childhood through adolescence, according to the study, and it drops again in late adulthood.)

“Everybody kind of thought it would be declining through middle age, but it doesn’t at all,” said Herman Pontzer, a professor of evolutionary anthropology at Duke University, who led the research.

Instead, Dr. Pontzer said, what feels like sudden weight gain when you hit midlife is more likely the accumulation of a pound or two a year over the past few decades. People just might not notice until they hit 40 and are 20 pounds heavier than they were in college. That gradual weight gain, he added, is usually caused by people eating a few more calories than their body burns every day.

Not everyone agrees with Dr. Pontzer’s conclusion. Susan Roberts, the senior associate dean of foundational research at the Geisel School of Medicine at Dartmouth, said some biological changes start to occur in midlife that can affect the body’s composition and metabolism.

First, the natural decline in muscle mass can alter how a person looks in the mirror, especially if they’ve added body fat over the years. That shift doesn’t have a huge effect on metabolism, though — a pound of muscle burns only about four more calories a day than a pound of fat.

A bigger contributor to slowing metabolism is the brain, which accounts for roughly 20 percent of the body’s energy use. Our brains start to shrink gradually in adulthood (more on this later), and less brain volume can mean fewer calories burned, Dr. Roberts said. “I don’t think the brain is the only piece of this, but I think it’s an important piece that is not really recognized.”

Why does it all seem to gather in my midsection?

Men and women both tend to complain about an increase in belly fat, in particular, in middle age. There is evidence that, for women at least, this complaint is warranted: For reasons scientists still don’t entirely understand, as hormone levels change with the onset of menopause, fat starts to accumulate more in the belly and less in the hips or thighs.

We turn from pears into apples, Dr. Faubion said. “That doesn’t translate to a change on the scale,” she added — the factors mentioned above cause that. But, taken together, it’s “kind of a little nasty combination for women.”

Perimenopause. What the hell?! Perimenopause.
What the hell?!

Perimenopause can catch women by surprise. Menopause is defined as not having a period for a year, and it hits, on average, around age 51. But women can experience dramatic fluctuations in their ovarian function and estrogen levels for up to 10 years before they stop menstruating — in some cases starting as early as the mid-30s.

Historically, this hormonal “roller coaster ride” of perimenopause was dismissed by doctors, Dr. Faubion said. “We used to tell women — I used to tell women — that, ‘Oh, you’re still menstruating regularly, you can’t possibly be having symptoms of menopause,’” she said. “But we now know that you can indeed have the same symptoms of menopause and be in perimenopause.”

Hot flashes and night sweats are the most frequently reported symptoms, but irritability, brain fog and feelings of anxiety and depression are also common. Many women experience disrupted sleep because of the night sweats, though Dr. Faubion thinks hormonal changes may contribute to insomnia in other ways we don’t yet understand.

Hormone therapy can help, and the risks of side effects are lower when it’s started early. Technically, the treatment is approved for hot flashes, vaginal dryness and the prevention of bone loss, but Dr. Faubion said that it can help with mood and sleep symptoms, too. In women who are perimenopausal and can still get pregnant, doctors may prescribe hormonal birth control to help regulate hormones instead.

Where did my libido go? Where did my libido go?

Both men and women can experience a decline in sex drive in middle age for a variety of reasons.

Why can it happen to men?

Sometimes, hormones are to blame. Concerns about low testosterone have received a lot of attention recently, and levels do drop with age. However, “most men will continue to retain normal levels throughout life,” said Dr. Shalender Bhasin, an endocrinologist at Brigham and Women’s Hospital in Boston who researches testosterone therapy.

Exactly how many men experience testosterone deficiency is hard to pinpoint. According to the American Urological Association, estimates span from 2 to 50 percent of men at any age, with rates for middle-aged men ranging from 4 to 12 percent. For men who do qualify as having clinically low testosterone, also known as hypogonadism, testosterone therapy can help treat their symptoms, which include decreased muscle mass, depression and fatigue, as well as low libido.

Whether testosterone therapy is beneficial and safe for men who don’t have hypogonadism is less clear, largely because the treatment hasn’t been tested in this population in a large clinical trial.

There’s more to libido than testosterone, though. According to one recent estimate, roughly a quarter of men aged 45 to 54 experience difficulty with their erection, and that percentage increases with age. Erectile dysfunction can make men self-conscious and less eager to have sex, said Dr. Alan Shindel, a professor of urology at the University of California, San Francisco.

Erectile dysfunction can occur for a variety of reasons — common ones include health issues like hypertension, high cholesterol and diabetes — and can be treated with medication or lifestyle changes, including exercise and a healthy diet. “Whatever is good for your heart is good for your penis, and it’s going to be good for your libido, too,” Dr. Shindel said.

Why can it happen to women?

Changing hormone levels in midlife can also affect women’s sex drive, albeit more indirectly. The few studies on the topic don’t show a clear and consistent correlation between declining estrogen during menopause and low libido, and treatment with estrogen hormone therapy doesn’t appear to increase sex drive, said Dr. Holly Thomas, an assistant professor of medicine and clinical and translational science at the University of Pittsburgh.

Women who experience frequent hot flashes and sleep disruption are more likely to report low libido, though, and the vaginal dryness that emerges for some women during menopause can make sex painful and, as a result, undesirable. Treating these symptoms can improve a woman’s overall well-being, and in turn, her interest in sex.

Psychosocial factors most likely play a bigger role, Dr. Thomas said. Research has shown that relationship quality, stress, fatigue and other health issues, including depression, all have a significant influence on postmenopausal women’s sex drive. “When you have mental bandwidth that’s being taken up by stressors, it makes sense that you may not have as much mental bandwidth to think about intimacy,” she said.

Psychosocial issues can plague men as well, Dr. Shindel said, though they may “have trouble accepting that there’s any psychosocial aspect to sexual concerns.” But, he added, “I always want to put that out there and kind of normalize it for men.”

If you’re concerned about your sex drive, talk to your partner and consider seeing a sex therapist. You can also ask your gynecologist or urologist to check for any physical health concerns.

And while libido does tend to drop off as we get older, Dr. Shindel said, “It never goes away. You’re never too old for sex.”

Is it just in my head, or am I already starting to forget things? Is it just in my head,
or am I already starting to forget things?

Your memory probably isn’t as good as it was in your 20s and 30s. But that’s just a normal part of brain development.

Brain volume peaks in a person’s 20s and then slowly shrinks through adulthood; this loss starts to accelerate in the 50s and 60s. Regions involved in attention, memory and executive functioning are especially affected, which in turn can alter some aspects of cognition, like how fast you think.

“The kind of memory changes that we see in midlife are typically not going to be indicative of some kind of dementia,” said Dr. Lachman, the Brandeis psychologist. “People might have something they want to remember, but it takes them a little longer to remember just because of the overall slowing of the central nervous system.”

While everyone experiences these age-related brain changes, how quickly they occur and how much cognition declines varies from person to person. Our health and behaviors — particularly exercise, nutrition, sleep, social connections and challenging yourself mentally — all contribute to healthy brain aging.

“This is the first window in time when we start seeing these separations across people, and that’s why it’s such an important period of time to understand,” said Gagan Wig, an associate professor of behavioral and brain sciences at the University of Texas at Dallas.

Although rare, early onset dementia does happen, and is possibly related to genetics. The primary sign that something may be wrong is if your memory is much worse compared to your peers, or if it declines rapidly, Dr. Wig said. If you’re worried your memory loss isn’t normal, or if you have a family history of early onset dementia, you should see your doctor.

On the brighter side, some cognitive processes actually get better with age — a change that’s “undersold,” Dr. Wig said. “Even though your speed might be decreasing, your verbal knowledge, and your world knowledge, and your access to semantic information actually increases.”

World knowledge comes from the information and experiences that a person accumulates during their life, he added. Some scientists refer to it as “crystallized intelligence”; others might call it wisdom.

Which health issues do I need to start looking out for? Which health issues do I need to start looking out for?

It’s not a coincidence that screenings for many age-related chronic diseases start now: Midlife is when our behaviors, toxic exposures and general wear on the body begin to catch up with us. (For example, decades of frequent small exposures to a carcinogen, like UV radiation from the sun, can cause gene mutations that lead to cancer.)

In women, menopause also increases the risk for several age-related conditions, most notably cardiovascular disease. Scientists think that’s because estrogen and other chemicals the premenopausal ovaries produce are protective for many of the body’s organs.

“While women have a biological advantage over men before menopause, some of that gap between men and women in terms of cardiovascular risk and metabolic risks closes after the menopause transition,” said Dr. Karlamangla, the U.C.L.A. geriatrician.

Some common health screenings, like mammograms and colonoscopies, are performed to detect a disease at its earliest stage possible, which can make treatment more successful. Others, like cholesterol checks and blood sugar tests, are intended to track how an aspect of health changes over time, so doctors can know if and when they need to intervene.

You should have your blood pressure and cholesterol checked regularly, even before you enter middle age. Get tested for pre-diabetes at 35 if you’re overweight or obese, and at 45 otherwise. Mammograms are now recommended starting at 40, and you should get your first colonoscopy at 45. Bone density scans aren’t officially recommended for women until 65, but if you have a family history of osteoporosis, you should talk to your doctor about getting screened earlier. Treatment of prostate cancer has become more nuanced in recent years, and as a result, so have recommendations around screening; ask your doctor for guidance. And if you’re a current or former heavy smoker, get screened for lung cancer starting at 50.