More a WWYD, rather than AIBU.
DS is 2.5 and has very slow growing hair. At the back it is now shoulder length, just about but at the front he looks like he has a receding hairline. The top is somewhere in the middle. I was similar at his age and have slow - normal hair growth now.
DH thinks we should cut it. I don't think it's necessary. He doesn't get mistaken for a girl (and what if he did?) and it's easy to maintain and doesn't get in his eyes.
DS couldn't care less. He has absolutely no option on the matter.
I'm very much of the opinion that kids DSs age have no choice in some things (teeth cleaning, being in his carseat in the car and medical stuff), limited choice in others (getting dressed, having nappy changed etc) and almost total choice in others (whether to eat the meal in front of him, what toys to play with out of the selection available, what TV show to watch during screen time). I think hair cutting falls in to the latter for me, unless it causes and issue for him that he cannot understand, which it currently isn't (no comments from peers etc).
DHs reasoning is simple - it's long, it needs cutting, it looks a bit unruly.
So we are at a stalemate. WWYD?