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.