Biologically they may be more or less fully developed but mentally most people at 18 and 19 aren’t going to be fully rational with their decision making, they just got out of secondary schooling and used to having everything done for them by their parents they aren’t going to be on the same level as a 30 year old. They may be ‘adults’, but still very young ones and nowhere close to being fully grown, at least mentally. It really depends on where you live but socially at least in the US they aren’t quite considered adults yet until 21. You may find this stupid but I don’t really care because 21, 18, 16, etc could all work for the age of majority you don’t go from child to an adult in day. 18 is a random number and people’s bodies and minds change differently.