Risk of them becoming unable to execute the duties of the office is a main factor. It's ok to discriminate against adults for the qualities of a high risk of developing age-related degenerative disorders over the course of their tenure.
This isn't that hard to understand. Your line of reasoning is pointless.
A bunch of people are living in the delusional reality of an old person and it's pushing us close to civil war for no good reason beyond boomer mass dementia. You have failed to understand the present crisis, so you fail to comprehend the necessity for this simple preventative measure.