Re: How old is Thor?
It's possible to get an approximate age for Loki and therefore Thor from the events depicted at the beginning of Thor.
Loki was born circa 965 A.D. we know this because the last war between Asgard and Jotunheim was shown to have occurred then, on Midgard (Earth). Odin and his army drove the Frost Giants back to their homeworld, where the All-Father found infant Loki abandoned to die in the temple. Thor is only slightly older than Loki, so even accounting for slower aging he must have been born just a few years before. Loki is therefore approximately 1,048 years old and Thor is slightly older.
Asgardian mortality was proven by the fact that Loki was able to kill Thor with the Destroyer. In The Avengers it was implied that impact with the Earth in that glass prison cell after a six-mile plunge would have killed Thor or Loki. Also, the Frost Giants who invaded Odin's vault killed the Asgardian guarding it.