My canon for my run is that Pryce’s Mamoswine (the one he has post game) isn’t Magnum evolved, but a different Pokemon entirely- Magnum’s kid, Haagen. Haagen, despite being stronger than Magnum, is still a daddy’s boy through and through and always listens to him… or at least most of the time. (If anyone wants me to give a brief explanation on how Gym leader teams work, I’ll do so)
So here’s the thing. In my Nuzlocke, physical age and evolution are, to a degree, completely separate. (For example, Hamma is canonically the youngest member of James’ team).
In the world I’ve established, Evolution is a process that happens naturally over time, usually at stages when the Pokemon’s body is ready for it. Some Pokemon don’t evolve out of choice, some evolve earlier, etc etc.
Hoooowever, with human intervention, battles for sport, trading, evolution stones and such, Pokemon can evolve much sooner than they’re physically ready. It’s not too big a set back, it’s just that the Pokemon’s body assumes they are at age X after evolving and the hormones pretty much scramble to catch up, which they do relatively quickly. Some cases, however, have the pokemon emotionally deal with the change poorly for a little while.