The walking speed of 45-year-olds can be used as a marker of their aging brains and bodies. The evidence was there in neurocognitive testing these individuals took at age 3 to indicate who would become the slower walkers. At 45, slower walkers have 'accelerated aging' on a 19-measure scale devised by researchers, and their lungs, teeth and immune systems tended to be in worse shape than the people who walked faster.