Brain scans from over 4,000 people, across the age range (9 months to 94 years) and including 1,385 Alzheimer's patients, has revealed an early divergence between those who go on to develop Alzheimer’s and those who age normally. This divergence is seen in early atrophy of the hippocampus before age 40, and in the amygdala around age 40.

https://www.eurekalert.org/pub_releases/2019-03/c-ahd030819.php

https://www.nature.com/articles/s41598-019-39809-8