The Evangelions are children, because they’re piloted by children. Children are small and light, so they can fit in the entry plug. The EVA is designed to be piloted by a child, not an adult. The EVA’s power source is the pilot’s own body, so if you’re too old, you can’t use it.
Children are also more malleable psychologically, so they’re easier to control than adults (in theory). That’s why they’re chosen as pilots; they don’t have enough experience with the world to question what they’re told or rebel against their superiors. Plus, teenagers have hormones raging through their bodies that make them more violent and impulsive than adults (and even younger children), which makes them perfect for fighting giant monsters and angels.
The actual reason why only 14 year olds can pilot evas is because there were no older test subjects available when the project was started and since then all other test subjects died in various ways before making it to 14 years old!
Last modified: September 25, 2022