From what little psychological studies I can find, that is basically true. Normally, underage relationships are illegal outright because one party is older and taking advantage. But even if both parties are underage, it is correlated with less emotional development and sometimes difficulty building other healthy relationships. It isn’t really an “ethical vs unethical” question when you have, say, two twelve year olds messing around. What are you gonna do, arrest them? Whip them? But if you’re some party responsible for those kids (parent, guardian, teacher, whatever) basically you want to keep them off of eachother lol. Hence why separate bathrooms, no co-ed sleepovers, whatever.