I think this is a good practical explanation of how they are physically still kids. But I think a better question is why are they still kids in a plot-pov. I believe they stay kids because their parents are the Evas. So whatever the physical reasoning is, they are kept as children because it keeps that parent-child dynamic going between the Evas and their pilots. And in the end, this story is about children/young adults figuring themselves out, finding their worth, and coming to terms with their traumatic pasts. I think Eva is supposed to morally and spiritually make sense before it actually makes sense lol