I turned 25 last year, and will be turning 26 very soon.
You will feel that way. It happens. You will start to notice little things about your body aging and it will reinforce that. You can still feel young. Learn to love life, finding little things that make you happy each day. Do something that makes you feel young. Have a tickle fight with someone. Go find some swings and swing on them. I work with kids which helps me a lot. It's about accepting the fact that you are getting older, but you don't have to give up enjoying life.
I don't mean to sound like an old codger, but 25 is a very normal age to start feeling this way. You are a quarter of a century old now after all
As far as the stuff from your past, you can't change any of that. If you want to pursue a higher degree at another college, set goals for yourself to get there. Another thing that has helped me is becoming more goal oriented. Feeling productive helps counteract the feeling of being old.
As far as mistakes as a teenager, isn't that what being a teenager is all about? Do you feel happy about who you are? I only ask because this could be more about self esteem than an age issue by itself. Your life experience has a lot to teach you, so don't regret and block stuff. Look at it and use it to better yourself, again being a productive person.
Realign your thoughts, not to see aging as "wearing out" but a "process towards perfection" or at least as close as we can get in each of our lifetimes.