Brains naturally lose white matter - the part of the brain that transmits information - as people age.
But a Cambridge University team found that loss was exacerbated with extra weight - so an overweight 50-year-old had a lean 60-year-old's brain.
Researchers said it shows we need to know relatively more about how extra weight affects the brain.
The team, from the Cambridge Centre for Ageing and Neuroscience, looked at the brains of 473 people aged between 20 and 87, dividing them into lean and overweight categories.