^technically they were too young for the age their characters were suppose to be with the backstory they were given (being preteen-teenages in the 40's) now for that back story to still work we'd actually need to be cast 80 y.o.'s so guess in a modern day setting (assuming that's even what they do) their back story needs an over haul, anyway
but, respectively younger versions of the characters are what we most recently saw in James McAvoy & Michael Fassbender so why go right back into that, at least it's been longer since we saw Patrick Stewart and Ian McKellan's "older" versions of the characters
and the characters being represented as older is more common in the comic's (tho, we have seen other versions, yes) that is the known representation of them