Some pilots are well suited to fly to their late sixties, others are not. How do you draw the line? Either you compromise and pick a fixed age limit or you find other ways to assess invidividual pilot's suitability. The latter will cost more money.
Screening for Altzheimer's etc. and assessment of psychomotoric fitness should at least be the norm for any applicant for class 1 medical over 60 (or whichever limit you choose) years of age. Sim checks are inadequate as one can often pass them routinely.
Most pilots will at some point of their careers fly with colleagues who shouldn't be flying anymore. The experience is usually not pleasant. I find the argument of possessing class 1 medical and having "right to work" unsettling, as the medical exam as of now does not cover for many age related issues. Also one is often unable to assess his performance objectively.
I welcome our senior colleagues to flight deck, but at some point of our lives the experience no longer outweights the ability and at that point latest the pilot must be let go. Either by enforcing a conservative age limit or by monitoring invidividual's mental and physical fitness at regular intervals more thoroughly than we do now.