One thing people keep questioning about the Mickey Mouse characters in KH is how Donald's nephews somehow don't age during the 10 years between BbS and KH1, but I've always personally explained it away as the passage of time not being the same between worlds, so I didn't mind it.
…what I do find difficult to comprehend, however, is how, in the proper Mickey Mouse universe, the nephews don't age while Max does.
There may not be a strict continuity between all pieces of Mickey Mouse media, but it's reasonable to assume the Upon a Christmas duology has a shared universe; Donald's nephews don't age at all, while Max goes from being a kid to a young adult.