It's not like nearly every superhero these days isn't played by an actor/actress who's in their 30s-40s.
The young X-Men of the original trilogy - Kitty, Bobby and Rogue + Cyclops, given Marsden is the youngest classic X-Men cast member, would suffice.
...uhhhhh...That's a flop bro. I love those actors and portrayals, but they are B-listers by the definition and could not carry an X-Men tentpole given the budget required and their individual price tags.
I don't think Anna Paquin/Ellen Page/Shawn Ashmore could sell a movie just with her name... if they ever headline a X-Men movie... the major selling points would be:
a. Its a X-Men movie;
b. The characters that they are portraying, characters that have mutant powers
I could definitely see them especially James Marsden mentoring The New Mutants squad.
And nobody seemed to care about Chris Pratt, Robert Downey, Jr, Chris Evans, Chris Hemsworth, James McAvoy, Hugh Jackman, Michael Fassbender, Tom Holland and whoever is playing younger Cyclops before they got their Marvel movie roles.
And I didn't say James Marsden could sell a X-Men movie by his name. So your point?