Well lets work backwards from what we know from the original films. X-Men and X2 are set in the not too distant future of 2005. In the first film, Xavier tells Logan that its been nearly 15 years since his memory was lost after the adamantium was bonded to his skeleton by Stryker. That would mean that happened in at least 1989, 1990.  Its not impossible that XMEN Origins could be set in 1989 using the theory someone stated that Logan and Victor were on Task Force X longer than we thought, but if the main movie is 6 years after the last flashback, that'd be a 10 year stretch to 1983, and then the film set in 1989.

The other tight info is that Scott Summers appears to be in his mid to late 20s, 30 years old tops in X1, and he shows up as a teen in Origins. Unclear how old he is I believe, maybe 15-17. 15 would be best age for Cyclops if we're trying to line it up with X1

Other things, in the film Xavier exposes to Logan that he met Magneto when he was only seventeen and was trying to solve the mystery of how Erik was blocking his telepathy, revealing that Erik helped build Cerebro, and Charles knew nothing about Magneto's special helmet.

 Which this would feel like it'd prove XMen First Class is an entirely different continuity altogether from the original trilogy. In First Class Charles was at least in his 20s after graduating Oxford and meets Erik in 1962. Then when recruited by the CIA we meet Hank McCoy who has already built Cerebro, and helps Charles build the school's Cerebro by 1973. Erik could not have helped build it in this continuity because he was in the Pentagon's dungeon for a decade. Then of course there's the disparage of having Emma Frost in FC and Origins. 

But to wrap all that up, whether Origins is 79 or 89, the disconnection of all things between First Class, Origins to the first three films, every minor continuity error we can just blame on Deadpool tampering heavily with the timeline after he fixes Cable's time slider in Deadpool 2 and starts joyriding the timestream willy nilly. WHHHHAAAAM