Plenty of retcons and stuff have happened since then, but thankfully most of it has been in the form of "Ragnarok cycles" introduced by Mike Oeming in the "Ragnarok" arc at the end of Thor vol. 2. That stated that the Asgardians have been manipulated into a never-ending cycle of death and rebirth that somehow empowers a shadowy pantheon of even greater beings called Those Who Sit Above in Shadow. Thor breaks the Asgardians out of that cycle with what he thought at the time would be one final Ragnarok (although obviously they're reborn again after that, thanks to Don Blake). But before that point, we can assume that the Norse gods existed in various forms for millennia and that, taken in conjunction with the story wobbly mentioned from Thor vol. 1 #293, the current version of Thor and the other Asgardians that we know and love has existed for the past 2000 years or so. So "our" Thor should be about 2000 years old. Maybe just shy of that, given that he'd have to have been born and grow up and everything.