An 11-year-old child is probably years from his or her first kiss. Yet the drug they are about to take will almost certainly lead to a medical pathway which will leave them sterile. Since their gametes will never be allowed to mature, doctors will not even be able to harvest their sperm or eggs. Can any 11-year-old understand the gravity of ruling out ever having children?
Moreover can this child, for whom sex is an unimaginable, probably rather revolting adult business, consent to a treatment which will depress their future libido to the extent they may never have an orgasm? (Imagine trying to explain the concept, let alone the desirability, of an orgasm to an 11-year-old.)
These are the ethical issues which make puberty blockers