Your body is like a machine—it wears down over time. But with proper maintenance, you can add years and quality to your life. Here are the numbers you should aim to beat.
Age 27: Muscle Mass May Start to Decline
After this age, men can lose 3 1/2 pounds of muscle per decade, according to a Portuguese study.
Protect Yourself: Twenty grams of whey protein after lifting can boost muscle gains 49 percent, say U.K. researchers.
Age 35: Crow's-Feet Emerge
Your skin's collagen is breaking down—from either smiling or squinting. If it's the latter, watch out: "There's a good chance that people with premature crow's-feet from squinting in sunlight are headed toward future facial skin cancer," says Neal Schultz, M.D., a Manhattan dermatologist.
Protect Yourself: Wear sporty wraparound sunglasses during outdoor activities like hiking or yard work, says Joel L. Cohen, M.D., a dermatological surgeon based in Colorado. "And apply sunscreen daily, even for your commute."
Age 43: Words Become Blurry Up Close
Hold this page out at arm's length. If you're over 40 and need to squint to read it, you may have presbyopia, a condition resulting from the loss of elasticity in your eyes' lenses, says Dennis Levi, O.D., Ph.D., a professor at UC Berkeley.
Protect Yourself: Again—wear shades. Sunglasses rated to block 99 to 100 percent of UVA and UVB rays and absorb UV up to 400 nanometers can help prevent eye damage. And if you're struggling on your computer, increase the font size. Every 2.8-point jump makes tasks seem 8 percent easier, say UC Berkeley scientists.
More: 6 sneaky cancer culprits
Age 45: Age Spots First Appear
Look at the back of your left hand, which receives extra sun while you drive. See brown or white patches? Those indicate damaged pigment-producing cells, which are reproducing too much, says MH dermatology advisor Adnan Nasir, M.D., Ph.D.
Protect Yourself: Whether you see age spots or not, apply an SPF 30 product every morning to all your exposed skin—even your arms—to help prevent skin cancer. If you want to lighten existing spots, use a product with kojic acid, like La-Roche Posay Mela-D Serum ($53, soap.com).
Age 65: Joint Pain Sets In
If your hands, knees, or hips hurt after exercise, you may have osteoarthritis, the breakdown of cartilage between bones.
Protect Yourself: Hit the gym. Overweight or obese people are nearly three times as likely to have osteoarthritis in the knee, the most common spot for older people. Already in pain? Try unloaded exercises, such as seated knee extensions. Japanese researchers found that these may be best for reducing joint pain.
Age 69: Hearing Needs Assistance
The high frequencies that sharpen speech drop out first, so people with hearing loss tend to think others are not speaking clearly, says Pamela Souza, Ph.D., CCC-A, a professor at Northwestern University.
Protect Yourself: Keep headphones below half volume. Listening to an iPod nano for an hour at just 50 percent can temporarily damage your ears, Belgian researchers found.