America’s aging population is posing special challenges, fitness experts say, because it is difficult to design effective workout routines for people with such a wide range of abilities.

For one 70-year-old, the goal may be to run a marathon, for another it’s getting out of a chair. Dr. Wojtek Chodzko-Zajko, an expert on aging with the American College of Sports Medicine says, “Some baby boomers could be athletic, while others would be unable to get out of bed.”

There are now more Americans age 65 and older than at any other time in U.S. history, according to Census Bureau figures. Some 40 million people age 65 and older lived in the United States in 2010, accounting for 13 percent of the total population. The older population grew from 3 million in 1900 to 40 million in 2010.

“Older adults should be doing aerobic activity to help maintain body weight, strengthening exercises to develop and maintain muscle mass, and some type of flexibility training,” according to Dr. James Graves, Dean of the College of Health at the University of Utah.

“Physical activity can reduce the risk of diseases such as diabetes, hypertension and osteoporosis,” he says, “as well as improve the quality of life by maintaining functional capacity, such as the ability to climb stairs, open doors, and carry groceries.”

“A very healthy 70-year-old can safely participate in high-intensity activity while a frail 60-year-old needs to lower the intensity,” said Graves. “My recommendation is to work with a personal trainer or group leader who has knowledge and qualifications to work with the elderly.” Of course, be sure to check with your physician before beginning any new exercise regimen.

Posture, breathing, balance, cognitive functioning and reaction time are among the most important—and neglected—components of elder fitness. Focusing on gait is also quite important because as we age our gait changes.

Other beneficial exercises include tossing a bean bag to improve reaction time, walking a figure-eight pattern for balance, as well as eye stretches, jaw relaxers, childhood games and cognitive challenges to keep both body and mind alert.

Experts agree that it’s never too late to do something. “Exercise is effective even in the most frail individual,” says Mary Ann Wilson, creator and host of “Sit and Be Fit,” a senior fitness program that has aired on U.S. public television since 1987. “If they can wiggle their toes, they can exercise.”