Ok, there are some comical old people out there, but I can't think of many.
Most comics lose it as they get older. Steve Martin anyone? A lot of the smarter ones retire or find something else to do. Michael Palin, for example, has reinvented himself as an "explorer".
Comedy works by surprising us. The longer a comedian is in business the less likely it is that we'll find his/her schtick surprising.
All comedy is subversive. Even the gentlest. It challenges things as they are. The older, more comfortable, more embedded in the establishment a comedian becomes the less unsettled and unsettling s/he's likely to be, the less in touch with the zeitgeist and the less essentially funny. Witness the career of Bob Hope.
The more you have to lose, the less willing you are to issue the challenge.
The comedians who last the longest are those who are funny by nature. Those who can't help it. Frankie Howerd for example. Frankie's comic longevity had nothing to do with his material and everything to do with who he was- that shamble, that long rubbery face, that unique combination of campness and misanthropic gloom.
The comedian is always a misfit. Out of kilter. Peculiar. Shamanic even.
Comedins lose it because they get scared. They get scared of the weirdness. They get scared of themselves.
Every great comic is a Yorick- that is to say, a death's head.