In certain ways, probably since I was a kid thanks to various fun little life circumstances. On a grander scale, I think somewhere between graduating from college for the first time, going to my second college, and moving out on my own only to realize "aww, crap, I've gotta pay all the bills now,". The situation at school is also a reality check from time to time. Although I'm not the oldest student by any means, I tend to be one of the older students in most of my classes and am lucky if I find others within a year or two of my age range. I got really lucky last semester and ran into two great friends, 24 and 26, who are both just coming to terms with the fact that they're adults now too.
Life is stressful at times because you don't always get what you want - I wanted to swing a summer semester this year but money was otherwise occupied by rent and bills and all that fun stuff. Despite those hassles that being an adult demands, it's cool to be more in control of my life than I've ever been and feel like I'm at a better place than ever before. I have a pretty clear outlook on where I'll (hopefully) be within a year or two and am finally fitting into a niche of my own in regards to what I want to do with my life.
But yeah, if you're not careful and try to grow up too fast, childhood will pass you the heck up and that's something you really don't want. Although there's a limited sense of freedom and a lot of growing, it's great to have that ability to make mistakes in your life and realize you have people to fall back on if things go south. Depending on your family and friends you'll probably have that as an adult as well but it's definitely a bitter pill to swallow if you realize that you're not quite ready to get out there on your own and be a contributing member to society as an adult.
But on the bright side, it's pretty awesome to come home and realize that you have this strange new place to call home thanks to your abilities as an adult and junk like that. Also it's pretty cool to indulge in things without judgement from others around you - I'm an avid book buyer, though I've had to tone that down lately in hopes of getting a decent savings going for myself.