If I'm not mistaken, I dont think you can take control over just one of those teams. You can choose to assign yourself to them or let someone else take care of them, but you will be the manager of the "main-team".
I don't think you can.. Unless you are the national A team manager and choose to take control of the U21's and/ or U19's. But if you retire for the A team I think you automaticaly resign from the youth sides.
The Way U do it is new user and put them as manager then sack the u19 or u21 manager and offer the main person the job accept it then retire the national player then u keep the job