i agree it is about maturity and mindset, not age. 18 may have made me legal, but it didn't make me a woman. i was still in school and i wasn't entirely comfy in my own skin yet. i remember that i couldn't even refer to myself as a woman at that point! lol
i think it's when you make that shift from trying to figure yourself out to actually knowing who you are and what you want from life and how to get it. for some, that happens in their teens and for others it comes later. i think for most of us, it's something that happens in the early to mid twenties. after having a little real life experience under our belts--complete with success, set-backs and failures.
for anyone in the "trying to figure it all out" stage, i'd call them a young woman. prior to that, you're a girl.
then again, who's definition are we using? even though my mom and i are friends now...she still thinks of me as a little girl with a lot of growing up to do sometimes
and i don't care how old i get or how much i go through, she is
always going to see me that way. in a way, i will always be her little girl, time and experience are not going to change that.