The hilarious thing about growing up, that all the ageist people here are gonna very harshly realise, is getting older doesn’t actually feel like anything. You don’t “turn into” an adult, it’s just another year that passes and, sure, it might become easier to make decisions or figure out how to fix a sticky situation but overall, you don’t suddenly Enter Adult World and never have a goofy thought or a messy moment ever again. You don’t just suddenly start reading the newspaper or hating music you loved the year before, or wanting to drink less alcohol, in fact you want to drink more because you’re old enough to know your limits and sometimes you’re old enough to say fuck it and go past your limit. There’s no set point where you magically start to feel Like An Adult, you just are one. That’s it. Nothing anyone can do about it. The only situation I could imagine someone feeling a very quick shift from teen to adult is losing a key family member and having to step up as a legal guardian for a younger sibling or relative. Or perhaps having a baby at a young age. Otherwise, you’re just slowly gaining more and more knowledge and understanding of yourself and the world and learning things in a different context to when you were 15, but there’s no monumental shift into Boring No Fun Adult once you pass 21, people will say “You’ve grown up” and you’ll be like “Yes ok in some ways, definitely, and in other ways I’m still very much a bratty 16 year old who wants to eat ice cream for breakfast″.
The secret to adulthood is everyone is just doing the best they can with what they know.