Research showed nearly half of a group of 18 to 26-year-olds had a gene mutation called presenilin 1 (PSEN1) making the disease likely at a young age.
Symptoms normally appear at around 45 in people with the PSEN1 gene, but the scientists found evidence 20 years before onset of the disease, the Daily Express reported.
The findings could lead to improved early detection, better clinical trials and more improved treatments.
It is thought recent drug trials were disappointing because damage to the nervous system is too extensive by the time symptoms are apparent.
Brain imaging, blood tests and analysis of cerebrospinal fluid (CSF) found 20 out of a group of 44 youngsters had PSEN1.
Researchers at the Banner Alzheimer's Institute in Arizona, Boston University and the University of Antioquia found PSEN1 carriers had more activity in the hippocampus, a key memory area of the brain, and less grey matter in some areas.
They also had more of a protein involved in depositing amyloid plaques in the brain
