Starting early is money in the bank, but even late bloomers can reap astonishing benefits.

"The game isn't over, even if you haven't been active," said Dr. Angela Smith, past president of the American College of Sports Medicine. "Aerobic fitness, bone health, agility, you may be able to catch up. It's remarkable to see the things people can actually do."

Smith, a physician at Philadelphia Children's Hospital, said studies have shown that even octogenarians can double their strength with weight training.

"There's good evidence that among people who have arthritis, the stronger have less pain, and that getting fit decreases the chance of having cancer," she said.

But if you're a former high school athlete who became sedentary as your temples grayed, don't expect your history to save you.

"Some of the benefits you built up aren't going to maintain themselves if you become a couch potato," Smith said. "That wonderful bone strength you built in your 20's will melt away a lot faster if you don't stay active."

Smith said logic dictates that 40-, 50- and 60-year olds need to pay attention to all components of fitness.

"Make certain to do flexibility, strength and aerobic training, even if you have to decrease the amount of impact. A jumping sport may be difficult. But many of the non-sport activities, swimming, weight training, yoga, people can do just fine in 40's, 50's, 60's."

Andrea Metcalf, a fitness expert and personal trainer for over 20 years, recommends bite-sized portions for the late beginners.

"People who have never exercised don't have a good sense of body awareness or body movement, so we need to focus on simple patterns, on strengthening those stabilizing muscles," said Metcalf, whose forthcoming DVD, Keeping Fit: Strength, Cardio, Pilates, targets the midlife exerciser.

"We know that 10-minute segments will have a cardiovascular effect. So if you do a couple, in the next week you can bring that up," she said.

Metcalf agrees it's never too late to start. "You'll feel the benefits within the first two weeks of doing any new program. In six weeks you'll see body changes. If you touch your arm it's going to feel stronger."

Dr. Wojtek Chodzko-Zajko, of the University of Illinois and the leader of the Active Aging Blueprint, a umbrella group which develops strategies to help people 50 and older increase physical activity.

He thinks people should revisit their routines just as they do their retirement investments.

"The 25-year-old kid that goes for a run without a warm-up will need to adjust that middle age, when he's more prone to injuries," Chodzko-Zajko said. "The choice of most 40-year olds is not going to be same as most 60-year olds."

He said studies in nursing homes have shown that strength-training residents increase their mobility. Some even get out of wheelchairs. "Even modest improvement is associated with huge differences in the quality of life."

Chodzko-Zajko urges people to find the activity that makes sense to them. "Even walking the dog or spending time outside. Adopt active choices. Be active in your own way."

Metcalf says people who live to be 100 all have a fitness component.

"Exercise is the fountain of youth. If you want to live to 100 you have to exercise," she explained.

So what's the best workout?

"The one you'll do," she said. "All the rest is just marketing."