So in secret wars, we will have the 3 main universes merging (the fantastic four universe,the main MCU universe and the new X men universe).

Obviously a massive chunk of Magnetos core as an person and who he is was experiencing the horrors of the holocaust, it's essential to him as an charcter.

But if the new X-men universe is set in the present day, even if he was 10 years old in 1945 (end of world war 2), he would still be at least 79 years old, we should have an younger magneto and Charles for the long term MCU.

So setting the X men universe in the 50's or 60's would solve that.
The new X-men universe has to be set in the past (50,60s etc...)