I am 30. I have 2 children, a mortgage, a failed marriage; one deceased partner, one ex husband, a lot of dead friends; sometimes very dificult experiences and a rather cynical outlook on life. I have been an adult for what feels like the whole of my life.
In contrast, my best friend lives with her mum still, travels a lot, has short lived passionate relationships and no children, and in general lives an utterly different life to me. We have known each other since we were young teenagers - in that time I have aged in a way she has not. We still look the same age, but behave very diferently most of the time. She is still young, I don't feel like I am anymore; not middle aged, but not young either.
I think that it is responsibility, rather than age, which denotes youth. The age at which we are expected to start taking on responsibilities has changed, largely I suspect due to different working patterns and oppertunities to previous years. I picked an old fashioned life, was a married SAHM in my very early 20s. My responsibilities aged me in a different way to the friends who went traveling and to uni, then worked in short contracts and unstable employment.