Mid-Summer Crisis

I'm having a bit of an adult-life crisis right now. It all started last week when the woman with whom I am staying commented that I get along so well with her daughter that I could be her older sister! This kid is one and a half. I thought to myself "Hell, I could easily be this kid's mother." Which made me think that I am probably about the same age as the woman and her husband which then led me to start pondering the meaning of being an adult and the social indicators that make people treat you like an adult.

I know that part of my existential crisis stems from the fact that I am resolutely in my late twenties and I am back in school and doing an unpaid internship, which is frankly beneath me given my job experience and skills. As a student (and an intern), I tend to get lumped in with the younger demographic, but even in my professional life I still got grouped with all of the other twenty-somethings who were either single or partnered-but-unmarried. So many people in our generation are marrying later or not at all and may not be choosing to have families but we lack any social mile markers for becoming an older and wiser adult other than marriage and kids.

I am feeling this acutely because I am spending so much time with people in the early twenties and like I said people treat us as being in the same life-stage. But we're not. I want and need different things, especially out of a job, than I did when I was 23. Maybe I'm wrong, but I feel if I was married or had kids, people would treat me more like the adult that I am (even though I am very aware that having a spouse and/or children does not in any way make you more of a grown-up). Since I am not interested in getting married or birthing any babies right now, are there any other social short cuts to separating myself from my younger colleagues, co-workers, fellow students?

Perhaps I am being paranoid about this whole thing. What do you think about all of this? How are you treated at work/at school/by your family? Does turning thirty help?