Young was in his prime as a winger for Villa and United, he has been declining ever since and he has shown he was much more capable for the job at left back than Shaw ever was.
Even before the leg break, Shaw was overweight and dropped by country and club and publicly slammed by Van Gaal. Shaw never wanted it enough, the only time he showed a glimpse that he looked like he was trying was when he was fighting for a fat contract but as soon as he got that he gave up and his performances dipped significantly.
You can question Young all you want but from age 29-34 (last 5 years as he was declining) but Young has shown more hunger for the role than a 19 year old Shaw when he joined the club to play a position that he's never even played before.