I dont see anything wrong with that, my best friend started T when he was 13 (in Alberta, Canada) after a year on blockers. Oh and, like other people have mentioned here, many people start their puberty before 13/14. I started developing breasts when I was 10, and got my period when I was 11.
Besides, if a kid show obvious sings of GID, I cant see why blockers (or even HRT) is wrong. Many GID kids have a greater sense of self than many cisgendered who are twice or even three times as old.