Because players who start playing for the first team at such an early age have a history of declining a lot earlier than players who break through at the age of 20.
By the time he was 18 he was playing for Manchester United in the champions league whilst also playing for England, being the main focal point of both teams.
Do they? I'm sure there are some, but there's plenty of examples of that not being the case.
Looking at other United stars, Beckham, Butt, Giggs, and Gary Neville all made their debut aged 17. None of them were going off the boil in their late 20s and none had declined anything like Rooney has. Giggs was playing to an incredible standard into his late 30s.
Some pretty massive revisionism going on here to pretend that Rooney couldn't have taken better care of himself or that's had nothing to do with his decline.