Let me rephrase my original statement. We clearly see that most girls in Russia at 13 are nowhere near an adolescent body type. Their body is of a pubescent child. Don't ask me why. Boys at 15 are usually in the middle of puberty -- they are adolescents -- and changes are already happening. They are way closer to being fully developed than a pubescent child of Trusova's type is.
Also, I am a woman, I know when puberty happens. That is why I think it's a little unnatural that there are SO many girls in Russian figure skating who look pubescent at the age of 13, 14, and even 15. For instance, I was already 167cm at 13 yo and easily passed for an 18-year-old with makeup. Kind of like Anna Pogo, actually (and I'm of the same nationality, haha). In contrast, the bodies of the girls in this "new Eteri generation" are so underdeveloped at 13 that there is no way I would ever give them any age above 12-13 no matter what they are wearing. This is a little unnerving.
Puberty age for girls vary a lot btw. While you could pass for an 18 years old at 13, many girls actually look their age. For me, Trusova looks like a regular 13 years old to me. I looked like that at 13, and most my friends looked like that at 13, so I don't really understand the "underdeveloped" part here, as you seem to take your own early development as average for Russia. The physical development pace varies a lot across different countries. There are also other factors that may affect it (for example, intense physical activity is known to delay puberty).
Anyways, as for development stage - in fact, the worst moment to learn and do difficult elements is when your body in in the middle of development, not yet a child body, and not yet the adult body. So if we use your argument that Sasha is still a kid and Nathan was in the middle of puberty, his case is actually more serious and dangerous. Basically its the worst moment: the bones grow quite fast, and muscles don't really follow, which, combined with physical strain, results in injuries.