Wasn't Kakashi in his early teens when he came up with that move? He was as old as Naruto was when he became a Genin. All minato said was that the move was unfinished and forbid him from using it since he couldn't quite keep up with the speed of it. Had Kakashi been older he probably would've over come that with just practice. The Sharigan just gave him a short cut to that and the same with Saseke.