Beyond the obvious, that Magneto was an older man when he first appeared, he hasn't been portrayed that way in the movies since the original trilogy. Michael Fassbender has been in the role for the last four as a fit, younger man. Ignoring the chronology of the new four movies making no sense, since they jump decades each movie but don't age, I don't think there's anything wrong with portraying him as older again. That said, the Holocaust was a considerable while ago and unless they slowed his aging because of a power, he probably wouldn't be a Holocaust survivor and able to take part in major action scenes. Even for mutants, they're just a branch of humanity and aging takes a toll.
I'm indifferent to race bending. It doesn't make me like a movie any less when an actor doesn't portray the comic race. It has to make some sense though. Some countries are notoriously white or all but single faith for many political reasons. In terms of Magneto, and his Malcolm X schtick, the message might be more powerful. However, it falls into the trope of a bitter black man as the villain to a righteous white man which is problematic representation on its own. Why not racebend Xavier? The white man doesn't have to be the hero of the story.
Conversely, racebending is a two way street. People can't campaign for white characters being changed and get affronted when it's done the opposite way. There are exceptions obviously, Johansson in Ghost in a Shell because that's set in Japan and is about a woman from the country it's set. Just an example, but it makes my point that when changing a character, logic should be applied. Does said country have that population? Are there plot points that make the original race important to the character and their story? For many characters, their race isn't important. It is malleable. Comics were done in a time when everything was very white and changes should be made.
On that note, there are plenty of diverse and interesting characters getting sidelined because the big names are just being changed so it's a double edged sword.
And I can't remember who mentioned Selene (who is apparently getting exposure soon) but she does have a national tie: she was originally associated with the Roman Empire. That's why she's called Selene and it's why she has ties to the Nova Roma colony of Brazil. I would say her skin should be a tad darker but then again, she's always portrayed as one step away from albinism.