…when you realise that you have to be the adult in your own life.
You are not just an adult, you are your own adult. Sure, your family will still give you help when you need it, your friends will support you, and authority figures will still be relevant (in a positive or negative role). But you can no longer eschew tasks that seem “too grown up” for you. You can’t expect people to sort things out for you. You have to at least try to do them yourself. So that when you need to, you can.
This is it.