To make a bad equivalency: would you have any reservations with someone younger than 18 choosing to have a vasectomy/tubes tied? What about 16? 13?
They don’t have the legal capacity to decide it, so the question is moot.
At some point we are going to agree that making that permanent¹ life choice isn’t a good idea as they just simply aren’t mature enough.
No, it’s just a question of legality. With your criteria, young trans people would be unable to decide that they need to start a soft transition because they’re not “mature enough”.
What the person you’re responding to is trying to get at, I think, is that many of these preferences or desires can easily change in your formative years as a young adult (18-25 for full frontal lobe development, I believe).
So? If it’s that important for them, they can reverse it, and if not, and it’s really that important, they can adopt. But less population is a net positive in general. One person is not going to change the general trend anyway.