Perhaps the most important factor in a fitness program as we age is having enough energy to maintain an active lifestyle. Due to the wear and tear of a lifetime of not taking care of ourselves, however, we may experience an energy crisis. Fortunately, there’s a way to refit and refuel.

We may think, "I can’t do what I used to do" or "I don’t have the vitality and stamina I used to have — how can I possibly stay fit?" Actually, the way energy is produced in our bodies is the very same whether you are 15, 35, 50 or 85 years of age. So energy currency can be kept at the maximum, regardless of age.

Your Body in your 30s

If you're over 35, it's time to pay attention to retaining your bone density and muscle mass. For every decade of inactivity after your 30s, you'll lose 10 percent of your muscle mass — more if you're inactive. You'll also gain fat and weight even if you don't change your eating habits, because calorie-hungry muscle burns more calories per kilogram than any other kind of tissue.

Now is the time to curb excess weight gain, which could boost your risk of health problems, including diabetes and heart disease, and could make you look and feel older.

Try these new strategies for minimising those risks:

Strength train
Think it's an odd time to start pumping iron? No way, finds a small but dramatic study of 25 women. Those who started strength training before their 40s saw bigger gains in bone density, which may prevent osteoporosis, than those who started later. Doing strength or resistance training twice weekly, is also your ticket to building calorie-burning muscle.

Jump
In your 30s, take advantage of the fact that your joints can still handle high-intensity aerobic exercises such as jumping rope and jogging. A recent study finds this kind of heart-pounding workout can actually slow the buildup of plaque in your arteries; the more intense the aerobics, the more marked the slowdown.

Weight-bearing aerobic exercises such as jumping rope do triple duty, helping you maintain strong bones and clean arteries while you burn calories. Whatever aerobic regimen you choose, aim for 30 to 60 minutes at a sweat-breaking pace four or five times a week.

Your Body in your 40s

Many women of your age are members of the so-called Sandwich Generation: with aging parents or in-laws as well as children under 21. Add to that the demands of a career, a spouse, and volunteer work, and you're likely to tell yourself, I'm too busy to exercise and eat right. But there are no good excuses for skipping exercise or eating poorly. By this time, you may have lost enough muscle to have slowed your metabolism, which means weight gain and heart trouble are real risks.

Take action without delay with these strategies:

Combine and conquer
Try circuit-training or new integrated-training class. These combine aerobics with muscle- and bone-building strength training in a single, highly-efficient session. Or combine family and workout time by walking, bicycling, or working out to exercise videos at home with your partner and kids.

Your Body in your 50s

From your mid-50s onward, age-related changes can make you lose muscle and gain weight more easily, especially if you're inactive. As a result of declining estrogen levels, you may find your new fat accumulating around your waist, not your hips.

Bad news, since research finds that apple-shaped women are at greater risk of heart disease than pears are. Hormonal changes can also make you lose bone faster, upping your odds of getting osteoporosis, and even fractures. The same strategies that helped you stay youthful earlier in life still work now.

So without further delay:

See your doctor
The benefits of exercise at this age are indisputable. A recent year long study of 173 inactive and overweight women ages 50 to 75 found that those who began exercising for 45 minutes five days a week lost four percent of their total body fat and reduced hard-to-shed belly fat. Another study tracking 140 women ages 44 to 66 found that those who enrolled in a one-year strength-training program boosted their bone density.

But, you must make a pit stop at the doctor before you get moving. You'll need your doctor's okay if you're going to start exercising in your 50s after a hiatus of several years, since you may have hidden heart problems, diabetes, and that sort of thing. If you've been cleared to exercise but have osteoporosis or heart disease, consider working with a trainer — at least at the beginning. Look for one who has experience with clients with similar health histories.

Loosen your grip
For strength training, choose machines over free weights. With free weights, you have to grip harder, and that can drive up blood pressure — a problem if you have high blood pressure.