Neuroplasticity, easier to control and also

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.

The Doylist answer is that Eva is a deconstruction of "super robot" tropes. Building-sized mecha having tokusatsu wrestling matches via remote control is a ten-year-old power fantasy. Like somehow only a child has the power, or only a child could be trusted. Taking that seriously gets you Oops, All Trauma.

Basically the "real robots" genre asks, how can we haggle down badass combat robots, to minimize the sci-fi bullshit required for them to be cool? That's how you get Gundam and especially Battletech. It's vertical tanks... with swords. War stories for teenage boys, who know 'I'll punch him harder' is not a universal winning strategy, but still think 'I'll just move faster' is. Meanwhile - Eva is Anno asking, what would the world look like if the absurd extreme cliches were just how things worked?

Basically it's The Boys for Mazinger Z. Ninety percent taking the premise at its word, a few big changes other series honestly should've done before, and a sprinkling of edgelord details like the vibro-knife being boxcutters... and that's how you get episode nineteen.

Cockpit's too small for a grown-up.

Because they run on childhood trauma.

Child soldiers just hit different

Because dad bod in a plugsuit is...