I'm not trying to tell you that your friends didn't try hard and that it wasn't a difficult process for them @Eeksteek But I do think that children should not be allowed to make these decisions.
When you say no one is trying to put a stop to children's 'sex lives', that's not the case. We have an age of consent - 16. Yes, children can decide to take contraception - but contraception is generally safe and well understood, and the lesser of two evils. A girl who gets pregnant at 13 or 14 could make the decision to have the baby, yes, and that would be risky, but at least we understand the risks and have good experience of mitigating them. In the UK at least, she would likely be encouraged to have an abortion. But with puberty blockers and hormones, we are running an experiment on children, and the early results are not looking good.
If you take a look at r/detrans, you will see post after post after post from very young people asking questions like whether their atrophied testicles will become normal again, or whether they will ever get their voice back, or how they can ever forgive themselves for what they've done to their bodies. It is heartbreaking.
I'm glad your young friend is doing so well, and hope very much that this continues to be the case. I hope that they won't regret that they can't have a sex life, or kids, and that they don't end up living in horrible pain. I hope they'll be well and happy for life. That doesn't change the fact that they should never have been allowed to go on experimental medication as a child. If an adult wants to medically or surgically transition, then that's absolutely their decision and they have every right to make it. They presumably understand the massive risks they're taking and what they have to lose. Children cannot understand this. If you don't know what it is to have an orgasm, how can you agree to never having one? If you haven't got through puberty, how do you know that you don't want your penis? At age 14, how can you decide you never want to have kids of your own? Children should not be allowed to make irreversible decisions like this.