You don't get many 18 year olds involved at under 21's though do you mate? I'd say he'd be close to the 19's though.
..absolutely, although the very exceptional ones (Rooney, Owen) can go straight into the full international side. Regarding Hornby, I just thought it was a point worth making and as @blueloon says, sometimes playing for other home nations can benefit the profile of a youngster.