When you turn 18 or 21
When your childhood fades away.
When you start doing your responsibilities and you put others (younger than you) needs first.
your own mother starts talking to you like you are one!
you actually move out, and are responsible for every aspect of your life.
random people start calling you ma'am or sir.
doctors stop giving you band-aids with cartoons on them. Also, when you can drive yourself to your own doctor's appointments.