I would say theres a spectrum, like with anything. But there are also many different spectrums that go into whether or not they'll succeed/grow as a defender.

Think of it like a video game and having "sliders" where you can turn their tendencies up and down. So some areas that will impact how much they learn:

Are they coachable?

If you arent receptive to the sender, the message will never get through

Are they receptive to playing defense?

This was me in my playing days. This is what I mean when I talk about being a "willing defender." I have talked at length about defense and offense and schemes and I can tell you exactly where someone should be in a number of different systems. But I wasnt the worlds most willing defender. So just because I knew where I was supposed to be doesnt mean I was always going to be there. Which is why Im talking to you as a former coach and not a former player.

How quickly can they shed bad habits?

Most kids in high school are so much better than their competition that it doesnt really matter if they do things the right way all the time. But when youre playing against 13 guys who are just as good as you, you cant have the same bad habits.

Can they process information and then cut their brain loose so they can turn it into action?

In football terms, "does their brain tie up their feet?" I coached a kid at (redacted university very close to VT) who was a supreme athlete. He is currently playing overseas in Germany. He had all the tools to be a great defender. But he absolutely could not process what we were telling him and turn it into action. He WANTED to, he was a great kid. He just couldnt. So you have to scheme around his deficiencies.