Published in the April 2013 issue

In our 20s, we feel real stress for the first time. We are in charge of our own lives, and that's a little unnerving. We have to pay bills. We might get married, or try not to. We have bosses, who are very different from professors. "You have to learn to decrease the effects of stress on your body," says Dr. George Kessler, an osteopath and attending physician at New York-Presbyterian Hospital. "Meditating even five, ten minutes a day — just letting your shoulders down and breathing — is hugely important. Stress is a very real, accumulative thing that may wind up increasing cholesterol and increasing diabetes risk, having you store fat more. Do you know what a telomere is? [It's a segment of DNA at the end of a chromosome that protects and stabilizes the chromosome.] There's good data that shows meditating might lengthen telomeres — that you might be able to halt the progression of a disease, or prevent it from declaring itself, by meditating."

The brain is made up of gray matter (living cells) and white matter (connections between the cells), and around your mid-20s, both reach maturity. This doesn't mean you can add two and two faster, but never in your life has information zinged through your head so quickly, allowing you to reason, decide, and moderate emotions like never before.

Your body, meanwhile, will set personal records in testosterone production. "Testosterone helps the brain in two areas: plasticity and resiliency," says Christian Pike, professor at the University of Southern California Davis School of Gerontology. "Plasticity is the brain's ability to modify itself, improve itself. This is important for learning and memory and adapting to problems. Resiliency is simply how strong the brain is. It affects how you overcome the increasing challenges to the brain as you age. Testosterone is important to both of these." The bad news is that some cognitive abilities start to slowly decline shortly after they peak, around your late 20s. But you won't notice this for a few decades.

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Exercise. Dr. James Galvin, director of clinical operations at New York University Medical Center's Comprehensive Center on Brain Aging, cites increasing evidence that aerobic exercise (running, biking) and anaerobic exercise (resistance training, like weight lifting) can have positive cognitive effects. If you've stopped going to the gym because you look fine in a bathing suit, start going again.

Eat a little better. "Heart-healthy diets are brain-healthy diets," says Galvin. Fresh fruits and vegetables, whole grains, lean meats, a little red wine.

Your immune system was fully formed around the time you hit puberty, according to Dr. Andrew Saxon, professor emeritus of clinical immunology at UCLA School of Medicine. There's not a lot you can do that will change it now.

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Assume that every person you are with is HIV positive, according to Kessler. "Act accordingly. And then you have to think about, is kissing safe sex? What if you brush your teeth before you kiss? Now you have microscopic blood. So mouthwash would be great, brushing no good."

If you do things like wash your hands (in general) and use condoms (for sex), you'll be fine. One thing you don't have to do is buy supplements that purport to boost your immune system. "Don't waste your money," says Saxon. "We vertebrates evolved from invertebrates 500 million years ago. Our immune system is built on the backbone of an invertebrate's, and it's robust. If you think drinking echinacea will help you, by that logic if you stopped drinking echinacea, it would kill you. The system is not meant to be pushed around like that."

You're a good 25 years away from needing a colonoscopy, or even having to know what your colon is. The enormous exception: If there's a history of ovarian, uterine, or colon cancer in your family, get checked now. And every few years. Sorry.

"The 20s are the golden age for your skin," according to Dr. Mathew Avram, director of the Dermatology Laser & Cosmetic Center at Massachusetts General Hospital.

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Melanoma is increasingly prevalent but treatable if detected when it's on the surface — the doctor cuts it out and you're basically good. Still, it's better not to get it at all, so wear sunscreen. Anything SPF 30 and above. "The difference between SPF 30, 50, and 75 is not significant," says Avram. "But use enough to fill a shot glass. Most people use less than that."

The bones, muscles, ligaments, joints, and cartilage that make up your musculoskeletal system are growing and need to stay active. "This is the time to build strong bones through impact exercise — your bones need to be bashed. And to build cardiovascular capacity not only by lifting with the muscles that impress women but through endurance training," says Dr. Vonda Wright, an orthopedic surgeon at the University of Pittsburgh Medical Center. "If you don't build a foundation in your 20s, it's hard to backtrack when you're 40."

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Exercise the rotator cuff (holds the shoulder together) using pulley or elastic machines at the gym.

Vary your activities so that different body parts build strength. For example, on different days: Pilates (increased flexibility, reduced joint pain, abdominal strength) then rowing (cardio, upper body) then cycling (cardio, leg strength).

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