Think it's a bit silly to try and force kids too far into one particular system, too early. You'd just end up with a bunch of one dimensional players who'd never experienced different ways of playing, they'd become completely reliant on that and have little or no problem solving ability, then when you find you need to change system for a particular game what do you do? None of the players have ever done any different. Then what happens if/when the manager changes and you end up with a new system for the first team, you'll have a bunch of 16/17 year olds who've spent years honing the old system and become redundant overnight.
Development football should be a lot more varied than first team football, results aren't important it's all about players learning, they need to play in different positions and different systems, encounter different situations where they can learn new solutions to the problems that they are faced with through this variation. Then as they get older and it starts to become more obvious where their strengths lie you can start to focus them toward a particular position, or perhaps still a spread of positions depending on the individual, it only needs to be right in the last stage of their step up to the first team that they really need to be indoctrinated with the first team tactics so they can make that final step up.