Hair: By age 55, at least 50 percent of men and women are about 50 percent gray.

Fit fix: Sorry, only lifting a bottle of hair dye can help this one.

Eyes: After age 45, at least 90 percent of men and women have trouble focusing at close range.
Fit fix: Wear sunglasses outdoors; if you work at a computer, periodically look away and focus on something in the distance: massage closed eyes with a warm paper towel or washcloth.

Heart: At age 25, the average man's maximum heart rate is approximately 193 beats per minute; a woman's, 195. Your maximum heart rate declines by about one beat per year. Max heart rate = 205 - 1/2 age (for men); 220 - age (for women).
Fit fix: Regular aerobic exercise, three to five days a week for a minimum of 30 minutes, will help keep your heart healthy.

Lungs: After age 25, the body's maximal oxygen consumption (the amount of oxygen it is able to absorb and use from each breath) decreases about 1 percent a year in sedentary individuals.
Fit fix: Regular aerobic exercise and resistance training will help preserve—if not improve—your body's efficiency and endurance.

Muscle: At 25, men start to lose six to seven lbs. of fat-free mass per decade; women, four to five lbs. per decade. At the same time, body fat increases by 2 to 3 percent for men, and 3.5 to 5 percent for women. Take the two together and you end up with a 5 percent decrease in men's and women's metabolism per decade.
Fit fix: Watch your diet and boost your calorie burning through exercise, especially strength training.

Flexibility: Men and women achieve peak range of motion in their mid- to late 20s. After that, flexibility drops due to joint and muscle degeneration and inactivity.
Fit fix: Stretch for a minimum of five to 10 minutes at least two or three times a week.

Bones: Most men and women attain peak bone mass somewhere between 25 and 35. After this point, women lose bone at a rate of 1 percent per year (men lose bone more slowly). Women typically lose up to 20 percent of their bone by age 65; men, 10 to 15 percent.
Fit fix: Weight-bearing exercise and circuit weight training (along with adequate mineral intake) can increase bone strength and mass.