Whilst there may be an argument for individuals to continue past a mandatory retirement age I do not for one second believe the medical and training assessment is capable of identifying those individuals.
Until the class one medicals are significantly better (and more tailored to the individual) and the individual training assessments are better than they are today there has to be an arbitrary cut off age.
We have all heard of pilots who've keeled over the day after a class one and we all know pilots that can put it on for the 2 days of a sim but would fall to pieces at 3am at 30 west whilst they've been half asleep.