I was browsing Thor's Marvel wiki article and found this passage:
Sometime in the 9th Century AD, Thor travelled to Earth to promote his worship among the Vikings.
We already know that he is immortal, but how old is he? He still looks quite young, while his father Odin looks older, with a white beard and all.
Is he older than 1200 years, or does time flow differently on Asgard?
Thor is really old. According to Wikipedia:
Like all Asgardians, Thor is not truly immortal but relies upon periodic consumption of the Golden Apples of Idunn to sustain his extended lifespan, which to date has lasted many millennia.
In the 2008 story The Truth of History we get a look back at a younger Thor as he visits Egypt during the construction of the pyramids. Those were built between 2630 and 664 BC, so that places Thor's age at least in the 2600 - 4600 year range.
In addition to that, I don't think time flows differently in Asgard than in the rest of the Marvel Universe. Thor has been away from Midgard for extended periods of time on several different occasions through the years (and many other Marvel characters have visited Asgard) with no mention of a time disparity.
In regards to the cinematic universe, it doesn't give a specific age, but one can do a little bit of inference using the official timeline:
(link to larger image here)
According to this, Laufey is defeated by Odin 1000 years before Tony declares "I am Iron Man."
Loki, then an infant, is taken from Laufey at this point
and Odin refers to Thor as his firstborn in Thor, so Thor is at least 1000 years old.
Odin: Thor Odinson, my heir, my first born. So long entrusted with the mighty hammer, Mjöllnir, forged in the heart of a dying star. It's power has no equal! It's a weapon to destroy or as a tool to build. It is a fit companion for a king. I have defended Asgard, and the lives of the innocent across my realms in the time of the great beginning.
