Magneto should be quite old, but not THAT old. What about having him tied to Vietnam war? He could be one of the younger american soldiers. Maybe captured and tortured? Some kind of trauma to wake up his dormant mutant powerHe'd have to be very old indeed to be a direct victim of the holocaust unless you slowed his ageing.
Maybe even turn the character asian? He could be a kid in a village in Vietnam, his parents killed etc