i think that this is a particular version of my ideas, but you, in my opinion, underestimate a "focus".
In a small country, like central american countries, or african countries (Ghana, Bahamas, Barbados, Senegal and others) the poor activity of the managers, and the poor players pool, may have, as result, a 21yo player which is better than older players. I had explained, some post ago, which is the best solution in the case just written, and i'm sure that this is the best solution because i was a scout of Ghana U-21 team, and Ghana improved only when the NT and the U-21 coach cooperates with common decision.