T he body’s gradual decline stems not from the passing of years but from the combined effects of inactivity and poor nutrition. So no matter what your present health status or your chronological age, regular exercise and improved eating habits will lower your biological age.

Benefits: Reduced body fat, increased muscle mass, strength increases of 200% to 300%, increases in aerobic capacity by 20% or more and reduced risk of heart disease, diabetes, osteoporosis and other age-related ailments.

Your goal should not be to become immortal but to remain healthy and vigorous for as long as possible and to compress the inevitable period of decline preceding death from several years into a few weeks or months.

To gauge your biological age: Forget how many birthdays you’ve marked. Instead consider how you stack up in terms of the 10 key “biomarkers” identified by our lab:

Muscle mass. As Americans move from adolescence into old age, we lose almost seven pounds of lean body mass each decade, a rate that accelerates after age 45. Reduced muscle mass leads not only to reduced strength but also to an increased risk of heart disease and diabetes, reduced aerobic capacity and a slower metabolism (which promotes gain of fat).

Good news: Those who remain physically active lose little muscle tissue as they age. All it takes is 20 to 30 minutes of aerobic exercise two or three times weekly.

Strength. Between the ages of 20 and 70, the average American loses about 30% of his/her muscle cells, including a large proportion of “fast-twitch” cells needed for sprinting and other high-exertion exercise. Unchecked, this loss of muscle leads eventually to sarcopenia, the severe, debilitating weakness that makes independent living impossible.

Good news: While we cannot prevent loss of muscle cells, a weight-lifting regimen will compensate by boosting the size and strength of the cells that remain.

Myth: Weights are appropriate only for young people and bodybuilders. In fact, I’ve watched frail 90-year-olds use weights to boost their arm and leg strength by 200% to 300% with just a few weeks of twice-weekly, weight-lifting sessions.

Essential: Ten repetitions of 10 lifts with a weight, leaving your muscles completely fatigued. If not, add more weight.

Metabolic rate. Because more energy is needed to maintain muscle than to maintain fat, the less muscle tissue in your body, the slower your metabolism and the fewer calories you must consume to maintain ideal body weight. Beginning at age 20, the average person’s metabolic rate drops about 2% per decade. Thus the average 70-year-old needs 500 fewer calories a day than the average 25-year-old.

Problem: Many middle-aged Americans continue eating as if they were 20. Eventual result: Obesity. To fight fat, eat fewer calories and get enough exercise to maintain your muscle mass.

Good news: Two to three days of strength training will boost your metabolic rate by up to 15%. Aerobic exercise, on the other hand, has very little effect on metabolic rate.

Body fat percentage. In most cases, advancing age brings not only muscle loss but fat gain. Even if our weight (as measured by a scale) changes little, the ratio of fat to lean in our bodies can rise markedly over the years.

Example: The body of an average 25-year-old woman is 25% fat, while an average 65-year-old woman is about 43% fat. For men, the numbers rise from 18% fat at 25 to 38% at 65.

Danger: Excessive fat leads to chronic disease and premature death and has been shown to be a strong predictor of physical disability in later life.

Especially dangerous: Fat around the waist. This is far more unhealthy than fat on the buttocks or thighs.

To lose fat and gain muscle: Combine a low-fat diet with regular exercise.

Aerobic capacity. To gauge fitness, doctors often measure the body’s ability to process oxygen during exercise. The greater this aerobic capacity, the faster oxygen is pumped throughout the body and the fitter the individual. Like other biomarkers, aerobic capacity often declines with age. Typically, by age 65 it is 30% to 40% below its level in young adulthood.

Good news: Regular, aerobic exercise, the kind that causes huffing and puffing, will raise your aerobic capacity, no matter what your present age. The longer and harder your workouts, the greater the benefit.

Blood-sugar tolerance. For most Americans, aging brings about a gradual decline in the body’s ability to metabolize blood sugar (glucose). So common is this problem that by age 70, 20% of men and 30% of women are at increased risk of diabetes, a potential killer.

At special risk for problems: Those who are overweight, sedentary and those who eat a fatty diet.

Good news: A low-fat, high-fiber diet, combined with regular exercise, will reduce your diabetes risk. Be sure to include both strength-building and aerobic exercise in your routine.

Cholesterol ratio. As most of us know, a high cholesterol level boosts the risk of heart disease. But total cholesterol isn’t the only thing that counts.

Very important: The ratio of total cholesterol to HDL (good) cholesterol. For older people, the ideal ratio is 4.5 or lower. Example: A person whose total cholesterol is 200 and whose HDL is 50 has a ratio of 200/50, or 4.0.

To lower your ratio: Stop smoking, lose weight, reduce your intake of fatty, cholesterol-rich foods and exercise regularly. Exercise is the only way to boost HDL levels.

Blood pressure. In many parts of the world, advancing age brings little if any change in blood pressure. In the US, however, where older people tend to be both overweight and sedentary, blood pressure does rise with age, often spiralling far above the maximum “safe” level of 145/80.

To keep pressure in check: Stay slim, don’t smoke, get regular exercise and limit your consumption of fat, salt and alcohol. If these steps fail, pressure-lowering drugs may be necessary.

Bone density. As we age, our skeletons slowly become weaker and more brittle. While some mineral loss is inevitable, the severe and potentially deadly condition known as osteoporosis is not.

Prevention: Although consuming at least 800 milligrams of calcium a day will retard the loss of bone, that alone rarely does the trick. Also needed: Weight-bearing exercise, such as walking, running or cycling.

Not helpful: Swimming and other forms of exercise that do not subject the long bones to the stress of gravity.

Body temperature regulation. Compared with young people, old people sweat less, get less thirsty and excrete more water in their urine. These seemingly minor changes, plus the loss of muscle tissue needed for efficient shivering, hinder the body’s ability to regulate its internal temperature, raising the risk of dehydration in summer and hypothermia in winter.

Good news: Staying fit keeps the “thermoregulation” mechanism in tip-top shape. Note: In healthy, well-hydrated individuals, urine is almost clear. Bright yellow urine suggests dehydration.