This goes back to my favorite Sex and the City episode, A Woman's Right to Shoes. Carrie's $500 brand new Manolo Blahnik shoes disappear after being forced to take them off at a baby shower for a friend's third child. When Carrie confronts the friend about the loss of her asset the friend chides her for spending so much on shoes, and refuses to pay to replace them.
I can't tell you exactly when a person goes from being a child to an adult, but I can tell you it's not always when they slip a ring on someone else's finger or push another human being from their uterus.
There's this idea that permanent relationships and children have to be a part of adulthood that is ingrained in us as children. It's not even an on-purpose thing. Our first impressions of adults are our parents, people who conventionally engaged in a marriage or long-term relationships and had children.
In many cultures, there are rituals to welcome children into adulthood (ex: Bar and Bat Mitzvah in Judaism ) But there are none in America. Legally, we're adults when we turn 18, but you still can't drink alcohol until you're 21, and you still need to submit your parent's income to financial aid for school, which is bull shit and subject for another rant. We're not expected to grow up until it's too late.
Financial independence, in my opinion, is true adulthood. If you make enough to support your lifestyle without relying on family money or credit cards, you have achieved adulthood. If your lifestyle includes a spouse and 2.5 children, that's great. If your lifestyle includes two cats, cookies and solitude, that works too, as long as you can afford it.
I'm not quite there yet as a recent grad, but it is what I'm working to, and I'm working hard to achieve it.
Then again, maybe we never grow up, maybe we just get older.