IME almost all 80 year olds can contribute much more than 20 year olds for any mental task, beyond super-demanding cognitive work like high-level math or some coding.

My point isn't the competition, it is the fact that there is a physical decline with age that no amount of training can offset, which you can observe if you compare to a 20yo. I think it is the same with the brain. And like for every large population it is a distribution, there are always outliers. But not many.