The medical risks of puberty blockers is still largely unstudied. Most I can find is a problem with bone mineralization and compromised fertility. They still aren't sure the effects they would have on brain development. The only resource I can find that isn't from bias anti-transgender websites that lists a study is pretty small. 20 individuals who took puberty blockers were compared cognitively to their transgender peers who didn't take them and the study found no determinable difference.

Also the hormones they use is transitioning are bio identical to human's natural hormones. The cells in your body only know what sex they should be based upon the sex hormones they bind too.

Also this charity in question, Mermaids, Doesn't even perscribe or offer any kind of hormone treatment. They are a resource charity that gives transgender kids and parents resources and counseling on how to handle gender dysphoria and transitioning. They are not capable of prescribing anything.

Teens are still in development as well, hence why they're teenagers and not adults; again, changing a person's hormones while developing can fuck them up for life, and it's not okay to teach kids stuff they shouldn't be dealing with at their age. All of this is just fucked up.