pointless comparing players with the likes of Rooney, Rooney was a freak at 16, basically fully grown then and already physically strong. 99.999% of other players aren't in that position, and it isn't hard to imagine how someone of Colwills build might pick up some niggling injuries after growing an inch or two.
I seem to remember Steven Gerrard having a late growth spurt that he struggled with for a season or so before bursting onto the scene when it had all settled down.