Originally Posted by psylockolussus
People didn't seem to have a problem with Famke appearing in The Wolverine even if her age is clearly showing.
Casual viewers won't really care about their age, they are already established to the X-Men characters that they portrayed in the previous X-Men movies. And its not like they look so fragile and not fit to wear costumes and do action sequences.
Remember Robert Downey Jr getting injured on the set of Iron Man 3? There kind of is an age limit.
I personally would use the Siege Perilous as a plot device for Cyclops, Storm and Phoenix. Then they're recast with younger actors. Gambit can die without Stryker's intervention from overloading with energy and then exploding, then he's cloned by Sinister and serves as a Marauder. Wolverine, Sabertooth and Deadpool have healing factors while Beast is covered in a ton of makeup so this is a non-issue for all of them. I'd probably de-age Storm first since she's the character with the most physical role. Cyclops and Phoenix always kind of take over the back ranks and viewers need to believe that Cyclops and Jean can be parents so I'd de-age them a bit later.
That or just use a sliding time scale like the comics and have Deadpool point it out. For instance, stating that Cyclops should theoretically be 60-something and Beast is WAY too old to be fighting on the front lines but it's okay since it's a movie.
Some mutants can also be said to have delayed aging.