Rom will get stronger. He might lose speed, like Rooney, but I bet Rooney is stronger now than when he was 20... He's certainly bigger.
Bale was a left back when he was 20, and Gerrard had made 8 apps for Liverpool at the same age. To suggest players can't or don't improve barely out of their teens is bizarre, and totally wrong.
You just aren't getting it, these players were technically competent. Physically they weren't mature, Gerrard was a late bloomer and thats why he had early back injury problems due to an excessive growth spurt.
Gareth Bale attended a training centre in 2007 for future stars with Micah Richards, the physically superior Ricahrds made Bale look like a child depsite him having all the tricks and skills, it took Bale to 2010 to get physically up to speed.
Lukaku was physically ready at 15, I spoke to Sharpie about him, his lad played against him for TNS and the TNS lads would not have it that he was 17 at the time, they were told he was that big a couple of years before then. A freak basically.