There needs to be a fixed retirement age and not just based on having a medical. It is not exactly difficult to find a doctor who will let you pass. Some geriatrics would keep on flying until they keel over otherwise, which is hardly good for safety. And no, I am not talking out of personal interests as I already have command. Whether that limit should be 65 or something higher is up for scientific debate, but better to retire too early than too late, just because you need to pay your 4 ex-wives.