TORONTO - No one likes to think about getting old and we generally don't do much to prepare for it. But the truth of the matter is that if you are lucky enough to live into your 70s, 80s and beyond, your fitness level is going to decline.

Some of it comes down to unstoppable biology. But how much and how fast you lose muscle, bone, flexibility and aerobic capacity is also influenced by your individual fitness level going into older age.

So if you want to be sprightly in your 70s, you need to be working out in your 40s, 50s and 60s, experts say.

Put another way: If you don't pay into your fitness bank in middle age, you won't have much to draw on later. And while you may not mind being too out of shape to go for a run when you are 55, you probably will care if you can't pull yourself out of a bathtub at 75.

"I think you are on a slippery slope. Or another analogy would be you're getting closer to the edge of the cliff," says Dr. Paul Oh, medical director of the cardiovascular prevention and rehabilitation program at Toronto Rehab, a hospital in the University Health Network.

"We need to be thinking about prevention all along, but particularly as we hit our middle years."

In your teens, 20s and 30s, for most people working out is about looking and feeling good, managing stress and keeping weight in check. But later in life, maintaining muscle is critical for independent and active living. In other words, we need to do it to be able to perform myriad functions we all take for granted — until we can no longer do them with ease.

You need strong leg muscles for walking, climbing stairs and getting up from the sofa. You need strong core muscles to protect your back. And you need upper body strength to carry groceries, push yourself up out of bed, open a jar or pick up a grandchild.

But adults begin to lose muscle mass as early as age 40, "so it's important to try to defend it through our adult years," says Oh.

In the U.S., the American Heart Association and the Centers for Disease Control recommend adults do weight training — with good reason, says Kent Adams, director of the exercise physiology laboratory at California State University Monterey Bay.

"For the boomers, critical to successful aging is strength training, resistance training,'' he says, adding that even people who exercise regularly but who focus exclusively on aerobic workouts should broaden their routines.

"Running alone is not suitable for maintaining muscle mass," Kent says. "From a public health perspective, we would do a lot of good if people would lift weights two or three days a week."

"We don't need to become Arnold Schwarzenegger.... But we do need to challenge ourselves and work harder."

Elaine Cress agrees.

"For a long time, the emphasis was all on heart disease and cardiovascular fitness. But people are really realizing the critical importance of having strength training for precisely this," says Cress, a former professor of kinesiology and gerontology at the University of Georgia who now lives in Bellingham, Wash.