Whats the difference between how a 30 year old is written vs 40? It all relative. Banshee, Forge, Wolverine are all written as their peers too despite being older.
False memories of a character at their current age of lets say 25 is not the same thing as real experience of another character at age 35. Scott and Jean didnt just wake up ad gain memories. They were transported to the future where they lived for over a decade and came back to the past. Something like this just came into play with Moira who is now considered several hundred years old bc each time she was reset back to a baby it counted despite us being reintroduced to her as a woman physically in her late 40s. Another example is Illyana who was 5 and went on to live 8 years in Limbo and come out a 13 year old despite only a few minutes passing. Thats more analogus to what happened with Scott and Jean
I dont think that aged him though as Mutant X occurred over a course of a few months and he was not returned to 616 at the point in which he "died". By the time Alex was found, a few years had passed. He aged naturally. The thing unique about Scott and Jean is them returning moments after they left. Its the same thing that happened with Illyana