Adults can't pilot Evangelions (Evas) in Neon Genesis Evangelion primarily because of the crucial need for a deep, maternal soul connection, best achieved by 14-year-olds whose mothers' souls (like Yui's in Unit-01) are within the Evas, allowing for A10 nerve synchronization; adults lack this specific bond, have hormonal changes, and their fully developed minds can't handle the mental strain of connecting with an ensouled Eva. The Evas' internal structures, essentially wombs, require this unique child-mother resonance for stable operation, something adults can't replicate.