Keep his origin unchanged.
As others said, let his powers explain his age (or lack thereof). Easy enough. Like, his powers allow him to actually and miraculously benefit from those magnetic bracelets. Boom, done.
I must say, though, I am still as fascinated/puzzled as ever with the apparent obsession with and assumption of this idea that mutants and the X-Men will come from another universe. That is not necessary at all. They provide an allegory of the racism and oppression in our society. It's no stretch at all to say that we haven't seen mutants yet because they were hiding, keeping a low profile, not relevant to previous plots (so they didn't need to be featured in previous movies). Or, the X-Men could have been active for years, but in secret. As colorful and explosive as they can be, Xavier can just sit back and mind-erase witnesses to cover their tracks. Again, easy explanation that doesn't break or muddle the allegory by making them come from another world.