When you say he gets caught out of position, I assume you mean when teams counter us, however everyone complains that our fullbacks don't get forward enough, so I'm not sure you can blame Young for this. It happens a lot in modern football, and the defensive midfielder is supposed to cover for the fullback when they go on attacking runs.
I don't agree with your second point either. He is probably the best crosser of the ball in our squad, and as recently as the FA Cup final he had Moses on toast, and breezed past him on pretty much every single occasion. Young is probably the most underrated player in our squad.
I'm not saying that he should definitely start next season, as I'd quite like us to have a natural left footer playing at LB, however I don't think its as simple as getting any old player that has played left back their whole career. I think it's more important that we find a replacement for Valencia, as his crossing is terrible, and struggles to beat defenders. (Last post of the day so wont be able to respond!)