It's like James Bond. He's eternal.
But a good way of addressing that concern would be to make the movie a period piece in the 60s or 70s.
And while I'm at it, I think one good way of making more James Bond movies that are more credible would be to do the same thing. James Bond was created for the 60s (and maybe the 70s), let's make all the James Bond movies into period pieces. X-Men First Class and X-Men DOFP proved you can do credible period pieces for that era.
But a good way of addressing that concern would be to make the movie a period piece in the 60s or 70s.
And while I'm at it, I think one good way of making more James Bond movies that are more credible would be to do the same thing. James Bond was created for the 60s (and maybe the 70s), let's make all the James Bond movies into period pieces. X-Men First Class and X-Men DOFP proved you can do credible period pieces for that era.