Yeah, I think qualities like varied sentence structure and word choice are things that should be learned later, and really only by those interested in the topics. My advice isn’t necessarily for younger writers; I’d also rather not distract our otherwise creative youth from the much more rewarding work of *creating*, as such technical aspects of craft could well be off-putting. That said, at some point discussion of technique becomes appropriate, similar to how at some point most musicians will study theory. (True, a few seem to get there completely organically, but such cases seem more like rarities.)
Personally, as an old cranky bastard these days, I’ve written and read enough to know what I like and do not like. As such, I tend to self-edit as I write, which isn’t necessarily good advice for everyone. But, I find I’m able to multitask when it comes to writing, keeping a broad focus, so to speak, not only on the story development, but also on the some structural elements of how it the text is crafted. Quite often, as you said, such aspects come naturally after so much experience. But, I’m usually aware of passages that need refinement, and usually improve those passages via techniques like these. We all, I think, gravitate toward what resonates with us personally as creators — -and that goes for content and style.