The Alzheimer’s gene, which dramatically raises the risk of developing dementia, is already affecting carriers by the age of three, shrinking their brains and lowering cognition, a new study suggests.

Children who carry the APOEe4 gene mutation , which raises the chance of dementia by 15 fold, were found to do less well in memory, attention and function tests.

Areas of the brain affected by Alzheimer’s disease, such as the hippocampus and parietal gyri, were also found to be up to 22 per cent smaller in volume.