Have to disagree with marvelamn, he WAS very funny years ago, with all the characters from his own series: The Slobs, Nice but Dim, stroppy teenager etc.
But some comedians do have a problem when they get older of still being funny.
Ken Dodd for example is still very funny and he is his his 80s.
But John Cleese, who is one of my all time comedy heroes, just makes me cringe when I see him now in the few things he is on.
Lenny Henry and Jasper Carrot are 2 others who are just not funny any more (not that Lenny Henry was ever that funny)
And I remember when Smith and Jones went past their sell by date. Some of their later series were just not funny.
They of course moved into more serious work, as did Terry Jones and Michael Palin from Monty Python.
A younger man in his 20s or 30s can still be funny being rather "zany" but it does not always work when they get into their 40s and 50s.
A comedian needs to "age" with their comedy, and being wild and wacky in your 50s does not always work.