I'm 28. I feel older all the time.
Part of this is physical differences. I have noticed it significantly harder to lose weight now, and am currently trying to get in shape. Sometimes when I'm having sex and I'm on my third or fourth orgasm my erections can become soft... that never happened when I was 17. I look older than I used to, even though with the right clothes/posture/haircut I can still pass for a teenager sometimes.
Some of this is differences in attitude. The old adage, you are only as old as you feel. Well, I do feel different than I did when I was eight. While this isn't as pronounced, because I think I still can be in some ways like a teenager: very impulsive, eager to try new things, open-minded about most things, flexible, easy going, etc.; there are differences. I'm more mature, less passionate, less prone to falling in love, less excited about new video games coming out or anything else for that matter. Nothing is as new as it once was.
Most of it, though, is just how I am treated. Age has never been a big issue for me when it comes to romance. I've been involved with girls many years my senior and other who were quite a bit younger than me. Yet if I flirt with a young girl now some people think I'm perverted or strange. When was this magic moment that I was supposed to become exclusively attracted to older women and how did I miss it? I was attracted to 19 year olds when I was 18 and that was okay but now that I'm 28 it's weird. I'm not sure how that makes sense. I probably get a little more respect for some of my ideas now than I used to, though when I was younger I was always told I was mature and well-spoken for my age. Now if I get compliments at all nobody mentions my age as part of the qualifier for the compliment. In a lot of ways I think and feel the same as I did when I was 16 or 18 (and in a lot of ways I have changed enormously)... but I never woke up one morning and said to myself "yep, I'm an adult now." I guess I get treated as though I am older than I feel, and this makes me feel older... I suppose eventually the two will catch up? Or maybe the perception of how I should feel will always outpace how I actually do feel.