It's set "Decades" after DOFP, So my first question was how old is Xavier now? I agree that it looks amazing, xmen meets The Last of Us, but yeah the timeline is looking rather confused, DOFP left Logan looking rather hope full in the new timeline. The issue is, DOFP had the events set "sometime" after the events of The Wolverine and the first 3 films, when Logan altered the timeline presumably he was returned to the exact same date but in the new timeline.
After a quick google:
1962 - First Class
1973 - DOFP past
1979 - Xmen Origin's The Wolverine
1983 - Apocalypse
2003 - X1 & X2
2006 - X3
2013 - The Wolverine
2023 - DOFP Future
Deadpool ignores the timeline entirely.
Xavier was born 1932, which means he was 91 during the film, if Logan is "decades" later then he is well over 100 years old during the film. The timeline would make more sense if for example rather than return to 2023, with the timeline changing as it did if Logan woke up in say 2006.