I think there was a canonical reason like brain-elasticity allowing the pilot to interface with the EVA (which was actually problematic in some cases so they had to install a 1 directional filter for the teen to pilot the EVA and not vice-versa).

Also, pretty sure the EVA are literally crafted from the hybridization of DNA and godflesh and uses the pilot’s own mother as a base. It’s symbolic of the mother protecting their unborn child from the horrors of the outside world, including from their own father in Shinji’s case, but ultimately failing and all the characters developing trauma as a result. So if it were adults the symbolism would be mostly lost.