At least with people in my circle of the US, it feels like there's an increasing infantilizing of people (frequently themselves, I'm a bit guilty of this), particularly in regards to things like marriage and raising a family. Yet, by 19 you'll have either made decisions for yourself or someone else has for you, that will likely put you on track for your entire future career direction (education, interests, college choice, maybe even enlisted in the military).

You don't need to have given "definitive thought" to your entire future to not be a child anymore.