What about this study?

Read up on the author. https://www.cmf.org.uk/resources/publications/authors/?id=294

The guy is clearly biased and immediately disqualifies himself by writing books about why evolution isn't real and why the brain must have a soul. He's no scientist, he's a religious fanatic pushing his own radical agenda. And if that's not enough for you, the linked study has factual inaccuracies in the abstract, such as whether puberty blockers have a positive impact on dysphoria. The fact is that statement misleads by suggesting that puberty blockers are intended to alleviate gender dysphoria, when the reality is that they're meant to prevent it from getting worse while other treatments (mostly social in nature, but also therapy and psychological evaluation) are meant to alleviate dysphoria and provide time for the patient to consider if those treatments are working or not, which informs the decision to continue on that treatment path.

No reasonable person is suggesting that children can make these decisions on their own. They're making the decisions with the aid of family and a whole team of medical and mental health professionals over the course of years and years. Nobody wants a kid to transition if they're not actually trans, because studies have shown that actually makes dysphoria worse.

The bottom line is that preventing a child from accessing that assistance is going to make them less able to make a healthy decision at the age of 18 than they otherwise would, with the aide of professionals, at the age of 15.