This seems to be the current guidelines in Britain. At least 12 for puberty blockers and 16 for cross-sex hormones, but only if he have been on blockers for a year at least.
Gender dysphoria - Treatment
I think it is definitely fair to worry about kids being misdiagnosed and that one should be adult before beginning their transition. I also understand why someone who is sure they are transgender wouldn't want to go through puberty before beginning their transition. This isn't necessary a new field in medicine, but it does feel like it is one that is expanding rapidly. Which is why it seems they are reviewing it constantly.
If I remember correctly, I believe 87% of the kids who enter the program over there do not transition. Many who like to fear monger like to use that number to show it all as BS, where I feel pretty confident in saying it is the exact opposite. That to means there is no rush to judgement. There is definitely none of the "trying to convince kids they are trans" idea TERFs throw out, but a genuine attempt to help everyone who comes in to the best of their abilities.