You're right. We should put all teenagers on puberty blockers until age 18. The high E or T spikes that occur with natal puberty dramatically change the body. And, we know that people absolutely can have puberty safely delayed for a long time. The AVERAGE age for girls to reach menarche was 17-18 before the industrial revolution. We actually have kids going through puberty at an age that is radically young by historical standards.

So we should put all kids, cis or trans, on puberty blockers. That way we don't have to worry about a cis kid mistakenly starting transition, or a trans kid not realizing they're trans in time and having to go through a natal puberty. Everyone just puts off puberty until they're done with high school. As a bonus, no more teen pregnancy!

/s

But really, the answer to your question is that it's poorly framed. We do let children, well specifically teenagers, consent to sex. But, just like trans healthcare, we do it in very carefully controlled settings. Teenagers are going to have sex. That's something that is simply going to happen. So in sane places, we have kids go through sex education. We teach them safe sex practices and teach them on healthy relationship patterns. And we try to encourage any teens that do choose to be sexually active to use protection and get on birth control. Oh, and we legally regulate teen sex without trying to ban it. We pass Romeo and Juliet laws that try to thread the needle. You don't want 40 year olds sleeping with 12 year olds, but you also don't want to put a 15-year old on the sex offender registry for sleeping with their 14-year old classmate. You use your brain, don't fall for moral panics, and try to design systems that navigate a very complex ethical landscape.

And really it's the same here. We're talking about people in the exact same age range - people 12-18. And we handle gender-affirming care for minors with a similar complexity and intelligence. We don't let 12 year old trans boys go on Amazon and order up vials of T shots. But we also don't infantilize trans kids either. We have them go through therapy and clinical assessment. We have clinical guidelines on what treatment is appropriate when. And most importantly, we don't fall for the naturalistic fallacy, which you have done. We recognize that just because something is natural does not mean it is good or desirable. Forcing a trans kid to go through a natal puberty is just as cruel and barbaric as dosing a cis kid with cross-sex hormones against their will. And we recognize that barbarity of telling 100 trans kids that they simply have to go through the trauma of natal puberty, just in case one of them happens to regret it later. In other words, we place equal value on the lives and well-being of cis kids and trans kids. You, however, seem to follow the rule of "it is acceptable for a hundred trans kids to suffer through Hell on Earth if it prevents one cis kid from mistakenly transitioning." You fundamentally do not place equal value on the lives of cis and trans people.

It works the same way that children approach other invasive medical interventions. If there's great harm and a costly intervention, then child, parents, and doctors work to decide if the intervention is appropriate. As a nice black and white example, it would be unthinkable to avoid giving a young leukemia patient chemotherapy where necessary.

Thus, the threat of harm has to be measured, and the danger of the treatment has to be measured. This happens. We understand that cases of gender dysphoria can cause real harm. These are only diagnosed when a child demonstrates a "persistent, insistent, and consistent" transgender identity. The doctors working with these children do the due diligence to ensure these are real cases with a strong need: the cases where we would be doing harm by failing to act.

Puberty blockers, despite unwarranted infamy, are excellent for buying a little time to ensure the proper judgement. These do not usually have permanent consequences. Other HRT treatments can be applied when we have high confidence that they are preventing harm. Again, that's pretty much like any other invasive pediatric treatment.

Underage sex and HRT are not really comparable.

That said I advocate for puberty blockers rather than HRT for adolescents myself. That allows them to safely postpone their decision if they are unsure and lets them decide when they are adults.

I'm not a doctor though, it's best to ask an endocrinologist (I think) to determine what's best for a child.