I agree. It is just that people's facilities fall off at different rates, but as others pointed out, it is hard to make an objective test that also can not be gamed.
There's nothing to prevent an older politician from advising their colleagues, serving on committee staffs or even lobbying. They're not prevented from participantion, they'd be prevented from being elected.
It's just that that's not actually really important. The test of a good system is one that admits that its members will retire and adjusts accordingly.
In the military, you're always training a replacement, and come 65 (or 68), you're retiring, regardless of your capacity.