Well, they can do whatever they want in these movies, but Magneto, as much "young" as he may have always looked, is the same age as Xavier, and they were children during the second World conflict, so the fact that now you perceive it as younger is just a fault of bad artists which started to depict him that way in the early 1990s. If you look for example at the rendition Kevin Nowlan gave of him in New Mutants #51, it is almost perfect, as he even aged him a bit compared to the classic image given in the silver age: