^ If Storm was 7 in 1962 then she'd be 47 in 2002, and probably older in X-Men given that it was set in the near future. Halle Berry will be 45 this August. Ergo, I guess that First Class can be seen as a partial reboot. There's really no obstacle to having people like Scott and Jean turn up.
Well, I think it's been hinted at that, perhaps, mutants age slower than humans. But the contnuity in this movie series is, indeed, a mess.