SEED is totally for the lulz. All the red coats (ZAFT elite) are kids below 18. I don't think they ever explained why.

Gunparade March, well cause the world has no much more adults as most were killed in the Shadow Beast wars so they have no choice but to use kids as their resource. Based on the strategy game with the same name I kinda like the theme to GPM, since they have to balance school life (pilots were regarded as social outcasts due to their time spent on missions rather than in the classroom with their peers) with their other duties. The same reason they used for Sky Girls. 99% of qualified able bodied combatants were killed in the WORM war.....of course needless to say they were mostly men. So now its time to use lolis. XD

Generic? Well yeah. Blame EVA for it. Its a theme that was unique at the time and it does provokes feelings for its target audience since they can relate between saving the world and their everyday school life.

I ma gonna assume those sexy suited female pilots are For Horny Men.

Those feature young school teens is to add the element of Harem or Love comedy.

Yaoi looking guys are for to attract fangurl.

And the rest is for the lulz.