For my son, it was between 12 and 13 years old. That did coincide with an injury that had him out of events and just conditioning for a while and I've wondered if that time "off" (just conditioning) prompted things for him, but since he has came back he has continued to grow (I was just noticing today that I have to remind him to shave before school tomorrow
). My son was shorter than his twin sister by an inch or more at their 12 year checkup and was several inches taller than her at their 13 year checkup. He grew about 6 inches in about 8 months during that time and started really developing muscle. He had gotten a tiny bit of underarm hair by the time he was about 11.5, but between 12 and 13 he full on got underarm hair. He started shaving his mustache just after turning 13. At 14 he has a mustache, some hair on his chin and cheeks and he is starting to really, really "bulk up". Gymnastics wise, it has been a good thing for him, he picks up skills much faster and got pretty easily got skills that other kids on his team have struggled with (like a press handstand on rings). On the flip side, one of his teammates is 16, same height as DS, barely has underarm hair (maybe he just isn't going to get it?), and while "cut", he hasn't bulked out muscle wise like my son.
I do think that kids this age are all over the place. My kids have a friend at school who is 2 weeks younger than my two. This kid was tall in elementary. In middle school everyone passed him. In the past 6 months he suddenly shot up 6-7 inches (about 5'9" now)! But I don't think that his voice has really changed yet, he still has no facial hair or underarm hair, and the doctors say that he still has a LOT of growing to do. Obviously he is later on the puberty side. He plays basketball and baseball, but not travel teams, so is not nearly as active as the gymnasts. He is just hitting puberty later