I think there's an incredibly tangible and obvious line for when a child becomes an adult. It's not the first alcoholic drink, loss of virginity, or onslaught of puberty. It's in the attitude. It's when a child starts to realize that his/her parents are people too, that they have needs, that they don't soley exist to meet the needs of their children.
This can happen too early in life. Kids can grow up too fast from many different experiences, including taking care of their parents. There's a scene that is a good example of this in Finding Neverland, where James Berrie (Johnny Depp) and George Llewelyn Davies, the oldest of the Davies boys, have this conversation in the theater:
George: Mother asked me to take the boys out for the afternoon. She said she only wanted a bit of quiet, but she was trembling so badly she couldn't even finish her tea. I'm not a fool, Uncle James. I deserve to know the truth.
James: I don't know the truth. She won't talk about it.
George: But you think it's serious?
James: It could be. The doctor felt she should go and have some tests.
George: Then you'll have to convince her to go then.
James: I've tried. She won't listen to me. And lately, to be quite honest, it seems that all my best intentions for your family have come to nothing but harm. Apparently I've made quite a mess of things.
George: It's Grandmother, isn't it? She's run you off, hasn't she?
James: Oh, she's absolutely tried with great effort. And perhaps with good reason.
George: It isn't you, Uncle Jim. She just... She just doesn't want to see Mother hurt anymore.
James: Look at that. How magnificent. The boy's gone. Somewhere during the last seconds,you've become a grown-up.
George grew up when he saw his mother as a person with needs and stepped in to help her.
Growing up too fast is sad, but it's also sad when people grow up too slowly. In our culture, there are many cases of kids selfishly relying on their parents into adulthood, without any thought to their parent's needs.
Parents can spend twenty-something years bending over backwards for their kids. Devotion to a child from a parent is expected, but the breakdown occurs when the adult kid turns a selfish eye and refuses to show gratitude, respect, or even help in some cases.
Ultimately, every kid must become an adult. When delayed, immaturity is made obvious.
Mother's Day is Sunday, followed by Father's day next month. If you haven't told your parents how much you appreciate their sacrifices for you, do it! Let them know you see that they're people too. They have needs for friendship, health, appreciation, etc. especially from their kids!
Stepping off the soap box now. Thanks for listening.
Note: Parents aren't perfect. I'm not saying that kids have to ignore their parents' faults. Also, this excludes abusive parents. That's a whole different, extremely sad situation.