...Ian McKellen, a man born in 1939, is too old to play a person who grew up in the horrors of ww2?
If you want to go by logical ages, all of the X-men should have been in their late 50's in the 2000 film. They've all kept a roughly similar age since 1963 compared to the other characters they encounter, and age at a glacially slow pace thanks to "comic book time". Magneto always looked like an extremely athletic 50 - 55 year old and at one point was even restored to that form after getting older. Even McKellen had doubts on playing the role initially, and didn't feel like he looked the part. He decided to move past that and play the character out of the way he could connect with him on some level.