It's quite funny that plinky posed this question. Funny because I guess I don't fit into the category. I don't think 'grown-up' is a one time certification that the society bestows upon you. I feel like a grown up when I'm talking to a not-so-grown-up guy. But then I feel like a kid when I'm talking to a more mature person.
I think of 'being grown up' as a role, a temporary one at that, that one assumes in certain situations. Mostly in my case it's the last resort because of the burden of responsibility it comes with.
So I guess for me the proper question that plinky should have posed is 'When did you last realize that you were an adult'