Of all the films in the X-verse, Deadpool 2 is perhaps the hardest to make fit, since it takes place 26 years after the events of Dark Phoenix — and yet during the brief glimpse we get in this film of Charles Xavier, Quicksilver, Cyclops, and all the rest, they appear to look exactly the same.
The only possible (if not plausible) explanation is that somehow, in resetting the timeline in 1973, the X-Men also somehow made it so they wouldn't show any signs whatsoever of ageing until, presumably, just prior to 2023, at which point they would rapid-age into the cast we see at the end of Days of Future Past.
Or, you know, this is just a Deadpool movie, and it's not supposed to make sense.