WARNING: FANBOY ANSWER
Yes. A) she's a mutant. For example, Logan (wolverine) fought in WWI because he doesn't age like the rest of us. Why is it so hard to swallow that Mistique ages differently? B)she's a shapeshifter. Who knows what she "really looks like?
They address this directly in the movie.
and Rebecca Rojmin makes a brief cameo, as does Logan(who's is hilarious)