To give the audience the sense of meeting the characters for the first time, in every movie.
From a 2014 interview with Simon Kinberg by Steve Weintraub for Collider on X-Men: Days of Future Past:
KINBERG: Yeah. One of the things, and this is an idea that came out of my work with Matthew Vaughn was, we didn’t want to do what most sequels do, which is pick up a day after or a month, or even a year after the last movie. We wanted to give it a big breadth of time so that you would meet these characters in some ways for the first time again, so we set the movie 10 years later than First Class ended and in doing that, part of my responsibility as the writer was creating a timeline so we can do exactly as you said, just give the actors a sense of who they’ve become and how they got there over the span of the 10 years we haven’t seen. And there’s a monologue in the movie that Hank has that Nick has, where he tells Wolverine essentially what happened. Wolverine comes to the mansion, he meets this broken, disheveled Charles Xavier and he says, “What happened to the Professor?” And Nick tells him what happened. “We started a school, it fell apart, he fell apart…” And all the things that happened over the last 10 years.
Since you mention the aging meme, Kinberg was asked about the aging in an interview on X-Men: Dark Phoenix with SyFyWire:
It's rationalization. Regarding Mystique, she can just make herself look younger, so essentially ageless. About the others:
But for all the other characters, Michael Fassbender, James McAvoy, Nicholas Hoult, going back to First Class, they certainly have not aged 30 years since we started making those movies 10 years ago," he acknowledges. "And the way that we sort of rationalized it, and it is a rationalization for sure, was that they aged a little differently because they're mutants, and we hoped that the audience would go with that. And so far that's not been a problem for the audience.