Who said that "18-year old" is the standard for sports just because "18" is a mark in many countries for administrative purposes? Why such preoccupation with "18"? girls transfer into physical maturity differently, individually, many factors affect.. Maybe "sports" have their own "timeline"? Maybe figure skating in terms of technical possibilities reached such a level where "the maximum" can be achieved between 13 and 16? Then why not admit that it is 13 to 16 for "high level competition"?
I don't buy an argument that "girls landing jumps diff. at 18 than at 12".... so what? why not say "girls land jumps diff. at 30 than at 18"?
Maybe "longevity" is overrated.. Perhaps some people don't "marry for life", "pick a sport for life", "choose a career for life"..... Perhaps people see their lives as a series of segments and stories, which start and finish hopefully satisfactory.
Perhaps one sees "sports" as just one stage in life and hopes to reach maximum, Olympic medal is one... And then go on to the next stage, college, career, other sports..... What's better than to win "the highest possible" between 13 and 15 (maybe 16/18) and to then go to the Best University in the country? Sarah Hughes, Debbie Thomas.. And to then (hypothetically) become the Best Golfer, or Best Yachtsmen, and then buy a series of properties and become a Real Estate giant? and then learn to paint or sculpt and become a commercial artist?
Why should "a given sport" have "longevity"? is that fans' wish? because it maybe NOT BE an athlete's wish... An athlete may want to just "get that Gold" and move on to the next Project in life.
There is still RIVALRY between US ladies, very strong rivalry..

. Perhaps what changed is that at the int'l level US girls at present are not "competing among themselves" for the top 3 places on the podium.
So what i am reading in your post is that you're missing "US ladies int'l podium domination"..... well, maybe russians heard once too often from american fans that "Irina is a truck driver" ..