Age restrictions for senior participation make less and less sense the more that it becomes apparent that juniors can do the same elements as seniors.
Only if the point of separating ages were about which elements the athletes do.
Swimmers all do the same strokes, but they are separated by age until senior level. (Although occasionally 15- to 18-year-olds have both gone to the Olympics and also competed locally in age group events.) Same for many other sports -- the specific skills used in competition are pretty much the same, it's the quality that differs.
It so happens that for rotational elements (jumps and spins), smaller body size is an advantage. So on those elements, and often those elements alone, at least the rotational aspects thereof, younger skaters may show greater difficulty than older skaters, especially in the case of teen girls.
In terms of actual SKATING across the ice on edges (the fundamental basis of what figure skating is all about, more important than rotating in the air or in place on the ice), on average skaters tend to improve on those skills as young adults compared to when they were younger teens. The best young teens may be better than the average 20-year-olds, but assuming they are still competing at 19 or 20 themselves they will likely be stronger SKATERS by then than they were at 13 or 15, even if they can no longer rotate as quickly in the air.
Case in point, Alysa Liu. And that's with a couple of years off in between in her case.
And then of course there are performance aspects that go into the program component scores. Again, the very best junior performers may be better than the average seniors, but for any given skater the performance qualities tend to improve as they move into senior age group.
This has been the case since 2006 when Mao Asada had to sit out of the Olympics, but if you're going to have juniors do identical required elements as the seniors then it's really pointless to prevent anyone good enough from competing as a senior.
As
@Mathman points out, the reason for dividing competition by age is not so much about seniors doing harder elements, but about the social/emotional effects of children competing against adults.
Where (what age) to draw the line between child and adult and how much if any overlap should be allowed between age groups are separate questions that have little to do with which elements are being performed.
If you need to artificially restrict the elements junior women are permitted to perform in order to justify the sport's age restrictions, it means something is really broken about your sport.
I don't think the point of restricting the solo axel jump in the junior women's SP is to justify the age restrictions. Those are two separate issues.
There are always exceptions, but in general women's rotational ability tends to peak in mid-teens, while their other skills are more likely to continue to improve into mid-20s and sometimes beyond. (Whereas for men, their jumping ability tends to improve with physical maturity, at least into mid-20s.)
It's realistic to expect that the very best female jump difficulty will be seen by the top juniors, that the average jump difficulty in the final group of a junior free skate is higher than that in the final group of seniors.
If these jumps are successfully rotated and landed, we'd expect the jump content to represent a larger percentage of the junior women's scores, especially vs. PCS, compared to the jump vs. PCS balance in seniors.
How that should be reflected in short program requirements is another question.
There's certainly a case for allowing solo 3A in the junior women's short program. It would not be a travesty to allow it. But it would separate a tiny percentage of girls who can do that element (maybe a couple dozen worldwide who give it a try, just a fraction of that amount at any given championship if those who try are concentrated in just a few federations) from the many hundreds of skaters who compete on the JGP and other junior competitions internationally and domestically.
At the top jumping level, it would also give an advantage to junior women who can do 3A but no quads a big SP advantage over junior women who can do one or more quads but not 3A.
For similar reasons, it would not be a travesty to disallow it for senior women as well.
At least until there are significant numbers of women/girls successfully performing these elements in free skates on a regular basis -- more than one or two or even five or six per championship.
Look at how many senior men were doing 3A or attempting quads as of 1998, before the SP rules finally changed to allow to allow a quad, or a triple as the required axel jump.
