Teen maturity is not uniform. Even adults make poor decisions sometimes. It doesn’t mean they stop bring able to make ALL decisions from them on, does it?
But children are children. They don't get to make adult decisions, because they're children. Adults get to make adult decisions for themselves, because they're adults. Yes, they can get things wrong and make bad decisions, but they're adults, so they knowingly take that risk. Children are not adults. They don't get to make adult decisions and take adult risks. That's not fair.
We don't accept children making decisions about many important parts of life. They can't vote, get married, get a tattoo, drive a car, have sex, drink alcohol etc. We do this to protect children. The same should apply to medical decisions that we know have a high rate of regret and serious complications and consequences, such as never being able to orgasm, being physically disabled, being dependent on drugs.
It also appears to be very, very rare that people regret transitioning (of course it happens, just not often) which suggests that something is working to make sure these kids really do want to transition, no?
The number of people who regret transitioning is HUGE and rising. We have begun to see the first lawsuits initiated - many, many more will follow. (To give you a sense of numbers, there are currently 36,000 members of r/detrans, a reddit sub for detransitioners.)
You're right, these kids do really want to transition - they think it will solve all their problems. So they transition, realise they still have problems, and now they also have a whole set of new problems to navigate, like being in constant pain, or dealing with incontinence in their 20s.