I don't remember if they explained it in the movies, but yes, Mystique doesn't age like normal human beings (according to the comics)
I also thought that this was the best X-Men movie yet in the franchise. I thought X-2 was formerly the best, but after re-watching this series it became apparent that these X-Men movies don't really stand the test of time very well. however, this movie should just because it was very timeless and sort of a James Bond-meets-X-Men type of movie
Michael Fassbender as a young Magneto was the best for me. the young Xavier was also very well done
all in all, a very fun and light-hearted (despite the Nazi sentiments) movie. the only actor I didn't like was January Jones, playing Emma Frost aka the White Queen. instead of being such a bimbo, Emma Frost should have been far more regal and self-assured