That's if you're a 40-year-old woman. Men of the same age should be able to do 27 push-ups. By age 60, women should still be able to do six push-ups and men 17.

The New York Times calls the simple push-up the ultimate barometer of fitness. When you do a push-up, you lift and lower your whole body, using muscle groups in your arms, chest, abdomen, hips and legs. It's a whole body work-out in one movement.

"It takes strength to do them, and it takes endurance to do a lot of them. "It's a good indication of what kind of physical condition you're in," fitness pioneer Jack LaLanne, 93, told The New York Times. In the 1950s he did fingertip push-ups and once set a world record by doing 1,000 push-ups in 23 minutes. He still does push-ups today.

"You are just using your own body and your body's weight," Steven G. Estes, a physical education professor and dean of the college of professional studies at Missouri Western State University," told the Times. "If you're going to demonstrate any kind of physical strength and power, that's the easiest, simplest, fastest way to do it."

Push-ups also help older people withstand the rigors of aging. Seniors who do push-ups are less likely to be injured in a fall. Why? Muscle memory will help them reach out and break that fall with their hands so they don't hit their head. If they do fall, those who can do push-ups are more capable of helping themselves up, something that requires upper body strength.