There were some exceptional skaters who were strong in school figures at young ages. Carol Heiss and Priscilla Hill come to mind. (Freeskating may have been Heiss's strength, but she was strong enough in figures to make the US world team starting at 13, in an era when figures counted for 60%.)
On average it seemed that female skaters seemed to hit their peak around 18-20. But many quit as soon as they achieved what they thought was their peak because of the expense, in an era of strict amateurism. We don't know how many would have had Kostner- or Kwan-like careers if they could have afforded it.
Also, before the late 1970s ladies were rarely doing triples at all and those who did try in the 60s and early 70s tend to be the younger ones. It might not have been surprising to see a triple in a junior competition and none in the senior event. Even by the late 1980s it was really only necessary to have 3T and 3S and maybe one other triple along with good figures and good everything else in order to medal. Once figures were gone and a full set of triples became a defining criterion in the early 90s, then it seemed that skaters started tending to peak younger.
But injuries have more to do with training than with age for competing at senior level.
If jumps are important in junior competition, junior (and novice) skaters will train to master jumps they can use to win in juniors.
Even if junior competition forbade harder jumps and senior competition allowed them, if coaches believe that skaters won't be able to learn harder jumps starting after full physical maturity, they will start teaching them before to those skaters who show readiness and aptitude.
So you would still have 13-year-old training those difficult jumps -- they just might never get to use them in competition if the jumps aren't allowed before seniors and many skaters who had or at least were working on them at 13 have lost them by 16.
