C.S. Lewis, author of "The Chronicles of Narnia," once said, "You are never too old to set another goal or to dream a new dream."

Oliver Wendell Holmes, Supreme Court Justice from 1902 to 1932, wrote, "Men [and women] do not quit playing because they grow old; they grow old because they quit playing."

Evolutionary biologists and biomedical researchers from Harvard University agree. They've published a paper that shows human beings aren't made to take it easy as they age. In fact, you're made to remain physically strong and active.

It turns out that the body is designed so that physical activity later in life shifts your energy toward all the built-in mechanisms that extend good health.

A longer health span to go along with a longer lifespan is the combination you want to cultivate.

It works like this: In older age, physical activity turns on specific genes that kick out health-compromising elements and activate health-enhancing ones.

Exercise also creates micro-tears in muscle fibers, cartilage, and bone that signal your positively stimulated biochemistry to kick into repair mode, building stronger tissues.

And of course, physical activity prevents excess fat storage and burns calories.

But if you don't do anything to stimulate such improvements, you slide into progressively worse health.

Whether you're walking 10,000 steps a day, doing aerobics and strength-building, or playing a sport (or better yet, all of the above), you're helping your body accomplish what it's designed for: to live healthfully for as long as possible.