Key points from article :
Slower walking speeds in old age have been linked to a higher risk of dementia and decline.
Slower walking at the age of 45 is a sign of faster ageing.
Study's 1000 participants had physical tests, brain function tests, brain scans, cognitive tests in childhood.
Researchers tested the walking speed of participants on an 8m-long pad.
Brain scans showed the slower walkers were more likely to have older-looking brains.
Measuring walking speed at a younger age could be a way of testing treatments to slow human ageing.
The study was published in JAMA Network Open.