I think I'm seeing a few different "adulthood" points in my life. First would be legal adulthood at 18... and the semi-independence of college, since I was still mostly supported by my parents. Then college graduation at 22 and entering the "real world". Getting my own place and becoming financially independent shortly thereater. And then I'm looking forward to finishing school and starting a career, and hopefully getting married and having children. Maybe it's less some clear dividing line and more like milestones along a winding, sometimes looping path.
I do like the new term "Emerging Adulthood" that has arisen in life development/psychology fields to describe the post-adolescent, pre-independent person, usually encompassing 18-25 but could easily last into the early 30s.
I don't think I will ever reach adulthood. I feel like there will always be things I am too young to understand. I'm very childlike in many ways and I just don't see myself ever catching up. I'm really unsure how I'd define adulthood. When I was a kid I thought adulthood meant you knew the answer to everything...lol.