Re: How old is Thor?
Immortality in fiction often can have many different meanings. I take the Asgardians immortality to be very long lived/slow aging and very difficult(by human standards) to kill but not completely incapable of dying. Then you throw in the whole afterlife schtick with Valhalla and Hela and her realm and then even death doesn't mean the same thing for them as it does for humans. And that's not even talking about the Ragnarok/reset button.
Beware of Space Dementia: It happened to Steve Buscemi. It happened to Matt Damon. It can happen to YOU!