Studies have shown that even a 90-year-old can build muscle, so the half-century mark is a good time to retire joint-stressing high jumps and to start lifting dumbbells to build strength.
Dr. Wayne Westcott, co-author of the book "Strength Training Past 50," said maintaining lean body mass becomes harder with ageing.
"The average man in good shape is about 85 percent lean weight, organs, blood, bones, muscles and skin, to 15 percent fat. The average healthy woman has a 75/25 ratio," said Westcott, fitness research director at the South Shore YMCA in Quincy, Massachusetts.
"It's more challenging with age but if you do strength training you can maintain your lean muscle to about age 70," he said, adding that an older woman who doesn't resistance train will lose up to 10 pounds (4.5 kilos) of lean mass per decade.
Westcott places equal value on cardiovascular training.
"We recommend approximately 20 to 30 minutes of resistance exercises two to three times a week. Then try to have an equal amount of aerobic activity four to five days a week," he explained.
Westcott added that older adults, who are hitting the gym in increasing numbers, might want to avoid explosive, high velocity activities, such as high jumps.
In 1990 there were 1.9 million health club members aged 55 and above, while in 2012 there were over 10 million, according to a 2014 report by the trade association IHRSA (International Health, Racquet & Sportsclub Association).
Dr. Barbara Bushman of the American College of Sports Medicine said regular physical activity, rather than a sedentary lifestyle, has the potential to minimize the physiological changes that occur with age and inactivity, in addition to limiting the progression of chronic diseases.
"Older adults can benefit from exercise, and although absolute improvements may be less than for younger adults, relative increases can be similar," Bushman said, adding that older adults may take longer to make improvements.
At 54, Florida-based fitness trainer and wellness coach Shirley Archer noticed that if she did not weight train she lost lean body muscle at a faster rate. She also found it harder to get it back.
Happily for Archer, who has enjoyed running, cycling and hiking, her endurance activities remain unaffected by her aging.
"I feel that I have not lost any endurance," said Archer, author of the book "Fitness 9 to 5: Easy Exercises for the Working Week."
As people age, she explained, they lose muscle fibers that produce quick powerful bursts before fibers that are engaged in endurance activities such as running or cycling.
She said that is why older athletes, who cannot physically compete against younger athletes when it comes to strength and power, can remain competitive in endurance sports.
The ageing exerciser also faces longer warm-up and recovery times, as the body is stiffer and slower to heal, Archer said. And the burning of fewer calories means paying even more attention to diet.
Staying hydrated is also important.
"We need to be sure to hydrate even if we don't feel particularly thirsty," she said. "Hydration will keep all systems working much more efficiently - and even help keep our thinking clear."