I know a few octogenarians that are sharper than most people in their 30s. Unfortunately, these are the rare exception.
After age 65, the risk of Alzheimer’s doubles every five years. After age 85, the risk reaches nearly one-third. This does not count all of the people with senility from other causes (such as strokes, loss of blood circulation, or other diseases).
https://alz.org/alzheimers-dementia/what-is-alzheimers/causes-and-risk-factors
Backing up, you would then take it that it is 1/6 at 80, 1/12 at 75, 1/24 at 70, and 1/48 at 65.And going forward, that it would be 2/3 at 90 - and 100% at 95.
Obviously, a lot of approximatin’ goin’ on . . .
In general, geometric progressions flatten out and actually approach some sort of saturation point.