Rooney drilled in an early opener but debutant Shane Long shot past David De Gea for a surprise equaliser and it took Young, on his first league appearance for the Reds, to set up the winner with nine minutes left.
United made an impressive statement of intent from the first whistle as the bold, youthful line-up selected by Sir Alex posted a positive start. Although Paul Scharner managed the first effort of note, it was all United and there was no surprise when Wayne Rooney drew first blood - as he did in this fixture last term.
Rooney expertly flicked a Fabio pass out to Young before collecting the return ball and curling a left-footed finish beyond the reach of Ben Foster in the 13th minute. The Reds were in control with Rooney blasting wide after being afforded too much space and Nani casually lifting a finish wastefully over the top following good approach work by Rooney and Danny Welbeck.
Nani again shot too high when Foster could only pat down a Young corner but Albion worked their way back into the game ominously. De Gea made a flying stop to thwart Scharner, who had stormed onto Long's lay-off but, two minutes later, the Spanish keeper coped less effectively when Long decided to have a crack himself. The Irishman fired across de Gea when Chris Smalling was unable to prevent the shot from the left-hand side of the box.
Conceding an equaliser was harsh on United but the defence seemed to lose some concentration as Fabio's panicky header away allowed Chris Brunt to volley over the bar. And de Gea had to claw away a Somen Tchoyi cross that