Peak is 17-18 years old. It was that way even in the era of figures. Peggy was 18. Dorothy 19, Katarina 18 when she won her first gold medal. Most skaters get their peak technical achievements as late teens.
There are a number of ex-skaters who have injuries, not just the girls. Todd and Alexei had hip replacements. Dorothy has Osteoarthritis, Michelle had hip surgery, Tara hip problems. Then there are whole bunch who are just hurting anonymously. I posted that interview by the ice dancer a few posts ago and she said, everyone (boys and girls) have aching backs and knees. Another former skater posted here saying that they still experienced back pain and they weren't even a top elite skater.
No one was screaming that Michelle Kwan was too young. At 17, she was considered "mature." Even in 1998 season, she had a stress fracture, just like Trusova had this season. So doing triples is not that much safer than quads.