Why are child pilots used in the first place?
Heavy spoilers for the original series:

But if after a decade you haven't still watched the original series, blame on you.

TL;DR:


   Because their moms' souls are what makes the EVA alive. And the EVA activates when the child (pilot) is placed inside the womb (entry plug).


Since the background story is the same for the rebuild movies, we can assume this is unchanged for rebuild.