My first thought was that this was a product of trying to keep Magneto a holocaust survivor.  Which makes him, what, 84 or so?  So his kids would be in their 50's?

But didn't Marvel retcon them into not being his kids anymore during the whole 'X-Men movie rights issue' period?

Then I realized that it should be the opposite. Going faster should slow down his aging compared to everyone else.

It can be hard with comics sometimes. Someone asked me how old I thought Professor X was once. I guess, as a youngster, I equated "bald professor" with being in his fifties. That's probably because, as a kid, a lot of teachers were bald and in their fifties (or so it seemed).

But then others in the discussion thought of Xavier as being in his thirties.

Another guy I chatted with thought Reed Richards was in his mid-to-late fifties, but even with a bit of white hair, I thought of him as being in his forties.

Not that any of it matters necessarily, but I suppose I am (badly) making a point about perception.