Forty is the chronological marker of middle-age. Fifty may be considered the new 40 -- and while it's a terrific attitude in terms of staying fit -- it is not the middle of an average lifespan. Only a rare few live to be 100.

Turning 40 is an important time to evaluate your fitness level. Perhaps you look and feel fantastic because you've stayed fit and eaten healthy through your 30s. But maybe you're not looking and feeling so hot and it's time to dig in and change those bad habits for good.

Don't ignore 40 and wait until 50 when it comes to staying active. New research reveals maintaining good physical function past the age of 40 will help reduce the risk of stroke. People who scored in the top quartile on a physical function test (e.g., kneeling, bending, lifting, climbing stairs) after 40 garnered a 50 percent lower risk of stroke than participants with the lowest test scores. The results were independent of known risk factors for stroke.