It's a very interesting question and, of course, there can never be a clear answer.
But when the gymnastics age limit was lifted from 15 to 16, it changed the entire sport. The 1992 Olympics was dominated by 15 year olds. There was Butyrskaya at 19, Onodi at 18, Zmeskal 16. And a slew of 15 year olds, who took home most of the medals. The problem was not that the 15 year olds won, IMO. The problem was that there were very few gymnasts staying in the sport later than age 16.
When the age limit rose, only by one year, a huge change occurred. By the following Olympics, a multitude of 18 & 19 year old athletes were not only competing but medaling & winning. Today we have competitive medal-worthy gymnasts in their 20s right alongside the 16 year olds. That would have been all but unthinkable in 1992.
Today's gymnastics rules & scoring system have altered the sport for the worse in many ways, but IMO the age limit shift of only one year was a rule change that clearly turned out to benefit the entire sport. Are there 15-year-old girls out there being denied their shot at an Olympic gold medal? No doubt. But today's 15-year-old gymnasts know they have a chance at a far fuller career in the long run than those prior to the rule change.
So I would say that one measuring stick should be the typical retirement age. When you see too many athletes getting out too fast or after a mere season or two in seniors, too few athletes lasting past the minimum age allowed. That's a sign that the ideal age cutoff should be higher.
I don't like to watch the women gymnastics today .. lacks of artistry. So many incredible strong and muscular women and if I want to be sincere some of them are overweight.
I know you will kill me because of these sentences.