Lawrence gets a pass just due to shapeshifting. Hoult might get one too, depending on what route they take. If he's in furry blue mode for most of the film, then it's not like we'll be able to discern age there anyway. If not, presumably with the help of an image inducer or something like that, then they can just say it's meant to make him look younger.
But yeah, I say throw some gray/silver hair on Fassbender. I'd go easy on wrinkles though, because for God knows what reason, with all the advances in make-up and prosthetics these days, they just can't manage to do subtle aging. You can make someone look 50 years older than they are, but not 10-12. Case in point: Harry Potter and the Deathly Hallows epilogue. They all look like they're 55. They're supposed to be 36.