I've worked with thousands of women. Twenty-somethings to 50-plus career women. Anorexics and starvers to bingers and bulimics. Top athletes to women who were so obese that they could barely walk a block. For all of them, a customized fitness "prescription" became an essential part of their self-care.

Although women of every age reap tremendous physical and mental benefits from regular physical activity, age is a factor in how much and what kind of activity their bodies need in order to get fit, energetic, and strong. Here's a Milestone-by-Milestone description of a woman's physical activity needs.

Milestone 1: Menstruation to First Pregnancy

Women in Milestone 1, with their surging estrogen and burgeoning breasts, hips, and thighs, typically have 21 to 32 percent body fat, well within a healthy range. They can keep this optimal level with regular physical activity. And because there's evidence that heart disease and diabetes can begin to develop in adolescence -- even childhood -- being active now, and staying active, can help protect them from heart disease, cancer, and diabetes downn the road.

The benefits of fitness extend beyond physical health. Research shows that physical activity eases stress and depression in girls and that girls who play sports have a more positive body image and higher self-esteem than those who don't. For that matter, girls who are active in sports are less likely to contemplate suicide than girls who aren't; they tend to spend more time concentrating on their physical accomplishments and the fun of teamwork than obsessing about their weight. Another study shows that when preteens and teen girls get in just 30 to 40 minutes of exercise each day, they can lower by 30 percent their likelihood of developing breast cancer later in life.

Milestone 2: The Reproductive Years

Women don't get fat from pregnancy. Their bodies simply are gaining the necessary reserves on hips and thighs to fuel the extra 500 calories a day required for breastfeeding. Women who enter pregnancy fit and stay physically active throughout the pregnancy are less likely to gain excess weight, and therefore, they have less to lose after they give birth. And being strong can definitely improve your endurance for the marathon of labor!

Of course, once the baby is born, a woman has to work to lose that excess fat, and the older you are when you deliver, the harder it is. Then, amid kids, career, the partner, and the whirlwind of everyday life, women in Milestone 2 face a most formidable challenge: to make physical activity a regular part of their high-stress lives. According to one study, being a mom cuts a woman's time for exercise by at least 20 percent. But putting in that sweat time is essential.

Regardless of whether a woman bears kids or not, beginning in her thirties, she normally loses 1 to 2 percent of her muscle per year, slowing her metabolism. She can even begin to lose bone. Regular physical activity revs up metabolism, preserves muscle, and slows bone loss.

It also safeguards her future health. Regular moderate-to-strenuous physical activity reduces a woman's risk for certain female cancers triggered by estrone, a type of estrogen that increases with higher-than-normal body fat and is associated with an increased risk of breast and uterine cancer. If you do regular physical activity from 1 to 3 hours a week from your teens to about age 40, you'll cut your risk of breast cancer by 20 to 30 percent. Four or more hours a week can reduce the risk almost 60 percent.