Canon has already done both - deaged him AND said his magnetism somehow makes him age slower.
What they were struggling with when I mostly stopped reading (over a decade ago) was navigating how to keep the emotional resonance as raw as he transitions to a historical figure.
Marvel has a lot of immortal/long-lived/time-traveling characters. It delights in giving them really intense backstories, and using the cushion of "this was long ago" to keep that intensity from being overwhelming. Sure, this guy murdered an entire city during the Crusades, but the Crusades are so long ago that they're basically a fairy tale, so that can just be used as an exciting visual!
Now a days, when they want a character's tragedy to be raw and real, they usually try to keep the ties to specific events ambiguous, so that writers can loosely shift the link as time goes on - Vietnam becomes Iraq, which becomes Afghanistan, which will eventually become somewhere else. Because we connect more deeply to things that we, personally, still remember.
You can't switch the war that Magneto survived. Him being Jewish, and the fact that it was the Holocaust, specifically, have become too important to the character. (Even though he was a survivor of Romani WWII internment at one point, and wasn't even in WWII during his early days.)
So his formative tragedy was 80 years ago. But he's NOT like Wolverine, whose history is an exaggerated thriller/horror movie. He's not like Apocalypse, or Sinister, or Exodus, whose histories are bloody fairy tales that carry no weight. He's not even like Captain America, whose history is a funhouse mirror reflection of the present - a constant conversation about the "good old days" and what makes America what it is. But those are the people he's starting to be compared to, because those are the types of stories immortal characters get.
How does the death stay meaningful as he comes back from the dead for the 5th time, and we're told his blood is being constantly rejuvenated through magnetism? How does his pain stay real when he's been reverted to an amnesiac toddler and then given doctored memories after being reaged?
The challenge of Magneto isn't figuring out why he's still alive and fighting. The challenge is figuring out how to keep the emotional resonance of his story as you add in more and more fiction.