He's 23 - he just doesn't defend well, it's not about learning IMO.
Well, considering he isn't a full back, if he's going to play there it kind of is about learning really isn't it?
He's played as a full back many times - he hasn't learnt how to defend and some people just aren't good at things. Learning can only take you so far, your actual ability will dictate if you're any good or not and I think he doesn't have it.