No way to tell right now. If you go by what Avengers book Odin was around a million years ago and at the end of a cycle at that point. What that would mean for Thor's age is mind boggling. On top of that we know Odin remembers all the cycles forward from that time. Does Thor remember them as well? IF the Phoenix is his mother that makes him somewhere near that era. The 1500 year age came from the movie and has not been verified in the comic book. It is possible I have missed some corroboration though even if so it probably contradicted elsewhere. I am not sure about how the change from universe 7 to 8 affects all that as well.
Do we know if gaia is n ow officially no longer His mother, so does that make the entire alone against the Celestial retconned away now, as main purpose for gaia to be his mother was to allow him to stay fully powerful even while on earth?