You are not old. Not physically, mentally, or any interpretation of the word. In fact, you're heading into what could arguably be the most amazing decade of your life. Your 20s is when you really get to discover not only who you are, but who you want to be in this world. It's an exciting and sometimes confusing time, but it's so, so amazing. You're gonna love it.
I know I'm not old and I know I'm going into a great time of my life (Sometimes it feels like anything would be better than high school), but it's just weird not being a "kid" anymore. It's like, my whole life I was kind of just given free reign to do whatever, and if I started worrying about something, someone would step in and say "Oh, don't worry about that. You're a kid, go have fun." Now, it's like I have to get a job and do well in school so I can go to college and stuff. Everything went from "Do whatever" to "Plan the rest of your life" so fast. I have to figure out taxes and savings and all of that good stuff. Sometimes I wish I was like 10 again, because everything was taken care of for me. But, at the same time, I can't wait until my 20's.