I'm just going off what I've heard and read from people I'd assume to know a fair bit about it, but basically, yeah.
The majority of 16-21 year olds play age appropriate football and only once they hit 21 start making bit part appearances in senior sides. There's a reason we have under 16s, 17s, 18s, 19s, 21s and even under 23s as distinct categories away from senior football. Rooney made his Everton debut at 16 years old in 2002, then went on to play in 37 games for them that season. He then made another 40 appearances the following season.
He signed for United aged 18, and had already made 77 senior appearances across the three top domestic competitions, then once you included his international caps, he'd already made 90 odd professional appearances. He then made 40 or more appearances every year for United for 8 consecutive seasons, ending in 2012-13 where he made 37. By the end of the 2012-13 season, still aged 27, Rooney had made 479 professional appearances at club level, and over 80 appearances for England at senior level. Additionally, for much of his career he has been one of, if not the only real key player for whoever he's played for.
If you take Ashley Young, who's just a few months older than Rooney, as a sort of "normal" example, he didn't make his professional debut until he was 18, and only made 6 appearances in his first season. At the end of this season, Rooney had already made 77 appearances for Everton. Young broke through properly in his second, aged 19, and maintained that for his third season. However, Young made his debut at Watford in Division 1/Championship, and didn't play in the Premier League until he was 21. Had Watford not been promoted, he may not have made his Premier League debut until later still.
At the end of 2012-13, aged 27, Ashley Young had made 355 professional appearances at club level, and just shy of 30 for England at senior level. That's over 170 games difference, and whilst Young was important for Watford and Villa, Rooney had been key for Everton, United and England for the majority of games he'd played for them by that point, in a way that just wasn't the same for Young.
In addition to playing a high number of games (719 at the moment, I think), Rooney's style of play has relied heavily on his physical attributes. It's no secret that players who rely on strength or speed tend to burn out sooner than those that rely more on their technical ability, and whilst Rooney's technical ability isn't exactly poor, it's never been something that particularly stood out about his game. I think this, and an inability to adjust his mental approach to the game, leaves him struggling now his legs aren't quite doing what he wants them to do anymore.
I think he did an interview once where he said every break from football, be it because of injury, a holiday in the off-season or simply a bit of a gap between games, would mean extra sessions in the gym for at least a couple of weeks to shed the weight and get his sharpness back, with more needed the longer the break. I think in the same one he mentioned Giggs not needing to do anything like that because his natural fitness was much better than Rooney's. Rooney's been playing top level football for 14 years now, so he's playing a high number of games, in a key role, and in a highly physical manner, for both club and country, as well as having to constantly do extra work in the gym every couple of months or so, since he was about 17. It's no wonder he's fecked.
