Well, I agree with respect to children ... if you're not old enough to vote, you're not old enough to decide to do something as radical as changing your sex.
But ... the brain is an immensely complex thing, and we know very little about how it works, or what can go wrong.
The fact is, there are people who are born in, as they feel, the wrong kind of body, and no amount of social pressure will change their minds. Some of them are gay, and not unhappy with the body they're in, but are not wired up to be attracted to the opposite sex. Why? No one knows. Others really are unhappy with the body they're in.
Now, when you're young, you go through all kinds of phases, are subject to strong peer pressure, are impulsive ... so requiring a 12-year old to wait before having radical irreversible surgery and massinve hormone injections is reasonable.
But ... what about a mature adult?
For example, what about the person who is now
Deirdre McClosky, but used to be
Donald McClosky, and didn't transition until she was 53 years old? Do you really want to give the state the power to prevent people like her from making the change? Our bodies don't belong to the state. (It's very interesting to read about this person's life:
[
Deirdre McCloskey - Wikipedia ]-- it shouldn't influence our position on this question, of course, but ... well, read the Wiki article, please.)