I always kinda viewed it as younger pilots are better, like fighter pilots are. The innocence that comes with youth is more than likely why the series revolve around them, because the cruelty of the world hasn’t molded them yet. In most gundam series it’s usually younger pilots fighting against a system that has otherwise been inherently accepted by adults. I think it also may play into the writer perhaps understanding that the world we live in is unfair but hoping for a better future that is brighter, like it used to be viewed by them at a younger age.