I recall seeing recently some psych research reported, showing that kids see everything as "black or white". The ability to see things in shades of grey (e.g. the lesser of two evils, best result for the most people, etc) and reason through ethical dilemmas doesn't really mature until about age 25.
This is why child soldiers are so valuable, they don't think about who the enemy is, the rights and wrongs of shooting at them and whether the value of doing so outweighs the risk to themselves, only that they are "the enemy" and must be shot at regardless.
It's also why if you have full membership at 14 and "one member, one vote" you end up stuck with Jeremy Corbyn as leader....