A lot of things in the body decreases when a person reaches their 40's—muscle mass and the thickness of the skin, which consequently leads to wrinkles. Likewise, a person's strength and energy starts to wane when he hits the big 4-0.
These reasons, among others, impel the 40-somethings to get into a rigid fitness regimen.
While that sounds daunting, these celebrities who turn 40 this year (gasp!) certainly make it look easy.