NEW YORK - America's ageing 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.

"If you are teaching 10-year-olds, it's perfectly reasonable to do an activity that everybody would participate in," said Dr. Wojtek Chodzko-Zajko, an expert on aging with the American College of Sports Medicine.

But 20 80-year-olds could be as different as chalk and cheese."

Some baby boomer could be athletic, he explained, 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 US history, according to Census Bureau figures. Some 40 million people age 65 and over 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 said, 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."