Marvel's unique approach to its official timeline poses some unique challenges for some older characters, perhaps none more than backstory is indelibly tied to the Holocaust. In Marvel Comics, the present day always matches the present day of the reader, with past events continually updated around that core fact. Thus, an event that happened 'ten years ago' for the character is continually updated, and could be depicted as taking place in the '60s, '70s, '80s etc. as time es. This works for most characters - the timeline of Spider-Man's origin needs to make sense with his current age - but poses a problem for those who are tied to specific historical events, since these can't move as freely as Marvel's internal canon.
Erik Lensherr's evolution into the terroristic Master of Magnetism began during World War II when he and his family were among the Jewish people captured by Hitler's forces and relocated to Vernichtungslager, a concentration camp in Auschwitz, Poland. There, Erik watched his entire family be systematically murdered except for him. Magneto's Jewish heritage and his connection to the Holocaust is an intrinsic and integral part of the character's story, both for himself and for fans, which leaves a conundrum for Marvel around how to address his age without stripping him of his history.
Fans often point out that given his connection to the Holocaust, Magneto should now be far older in Marvel canon than he's usually depicted, however Eric the Red, an evil Shi'ar agent, uses alien technology to rapidly age Magneto, leaving him at the peak of his powers.
Because of this, while Magneto may technically be over 90 years old, he is physically far younger, and has been for some time. At this point, most of Magneto's adventures take place after his age was artificially altered, explaining why he appears to be only a few years older than Cyclops and the other X-Men. A line was drawn under this process with the 'Krakoa Era' revamp of the X-Men line, in which mutants can be resurrected at the age of their choosing. This means there is one point in Magneto's timeline when his age was officially reset, and now a long period in which Marvel can later claim this happened as many times as it wants. While fans still debate whether Magneto's origin should pose a problem for his current age, Marvel has more than covered its bases.
Whatever Marvel's internal rules, Magneto should never be separated from his experiences in the Holocaust for storytelling convenience. Thankfully, Marvel's smart re-aging of the characters means there's no longer a problem to address, and Magneto's current age and past experiences can co-exist comfortably in future X-Men stories.
