i'm 27 and i definitely don't feel like a 'grown-up'.
and i don't think it changes as we get older either. my mom (66) and my grandma (88) have told me that they still feel like they are in their 20s. in fact, my grandma was looking at a couple that lives across the street from them and commented on how old they were- my grandfather had to remind her that they are probably around the same age as them!!!
i don't think you ever really 'grow up', my mom says you just get shocked everytime you look in the mirror and see how much you have aged!
The first time I felt like an adult was when I bought my car. I made the purchase without any help. My dad went with and people would go up to him and he would say "I'm not buying it you are talking to the wrong person." They always had this look of shock on their face to see someone so young purchasing the car I did. Other times people would comment, "is daddy buying you a car?" and I would just laugh and say no I don't need him to buy me a car- I can buy my own. When it came time to negotiate I did most of it. My dad said I was tougher on them than he would have been. I don't necessarily feel "grown up", but that definitely made me feel like and adult. I made that purchase all on my own and no longer needed my parents help- which to me was a huge deal.
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"Despite all your best intentions, sometimes, fate wins anyway."