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.

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I hope you'll take this to heart: Adopting a healthy lifestyle even in your 50s or 60s can significantly cut your risk of heart disease or premature death.

Researchers from the Medical University of South Carolina have been tracking data from nearly 16,000 women and men since the late 1980s and have determined that middle-aged adults who eat five or more daily servings of fruits and vegetables, exercise for at least two and a half hours a week, keep their weight down, and refrain from smoking cut their risk of heart disease by 35 percent and their risk of death by 40 percent after just four years. Impressive.

Essentially, these individuals caught up. They bought some time and ended up rivaling those who'd been doing the right things all along. Even those who didn't totally clean up their act -- perhaps they mastered just two or three of the lifestyle changes -- lowered their risk of dying.

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