Nearly every successful singles female to date competed at the top level by age of 17
Some exceptions:
https://en.wikipedia.org/wiki/Nancy_Kerrigan, still junior at 17
en.wikipedia.org
She may have been 17 at Piruetten, which I believe was a senior competition when she competed there. But not "top level."
(
This link's information doesn't quite line up with Butyrskaya's Wikipedia page. I think I own a print magazine that might have printed results, but it's not handy.)
Figures were a still a thing when they were juniors or starting to make the shift to seniors, so skaters tended to peak a little later. (And then, in the days of strict amateurism, often turn pro early if they hit it big, to make back some of the money they'd invested)
Less successful one-time world bronze medalist, still junior the year she was 17:
en.wikipedia.org
Again, skating is an outlet for prestige. Parents want their kids to be successful. They aren't spending hundreds of thousands of dollars and taking the kid out of regular school simply as a "hobby". The possibility of becoming an Olympian is the major driving force in dedicating all the time and money that is required. Without that option of the kid becoming an Olympian within a desirable timeframe, parents are not going to be nearly as enticed by figure skating.
The chances of becoming an Olympian in this sport, much less doing well enough there to have a career that will more than earn back the expenses of getting there, is vanishingly slim, the more so in countries with deeper fields.
For some skaters, some parents, aiming for the moon even though they know it's a longshot is worth the effort and expense, because of all the other rewards along the way.
For others, at some point it could be time to move on without having had any senior international assignments even when younger more talented (or just luckier) skaters were earning them, depending on their financial resources and their other interests and goals for the rest of their lives.
Some may have better chances if they're exceptionally talented from a country with a small field.
And some may just be deluded.
One size does not fit all.