Win the senior world championships, and you become an instant GM.
The current guy (Jens Kristiansen?) being the senior world champ, got his GM that way. So there's one 65+ becoming GM.
I think I've read about a 40+ dude becoming GM by collecting norms.
I'm sure that there are records of this sort of thing, but I don't know them.
When Ben Feingold became GM a few years ago, the cover of Chess Life had his picture with the caption "The 40 Year Old Grandmaster". I took that to indicate that achieving that goal at age 40 was considered rather extraordinary.
What would the oldest age be to get any FIDE titles based on abilities? Would you still be able to get an NM or CM after your prime? Could you still gain an IM norm at 40+?