Just because you are 20 it doesn't mean you are beyond the "teen" stage automatically. The rigid marker of what a teen is is misguided in many ways. People have different responsibilities and different freedoms at different ages. I know people who are 21 and still very much "teens". I know people that are 16 and are more mature than many adults.
I have lots of responsibilities and i can be mature in things that i do. But that doesn't mean i can't do things that i would have done when i was that little bit younger. Just take all the good things that you want in your life and work through the rest.
Lots of the time i find myself doing things that i am sure i would have done in my teens, i don't see much difference- although i do understand the feeling of loss that you describe. The good thing is that we can regain many of these things and empower ourselves (sure- some things can never be taken back- but regained in other ways).
I feel like I've missed out on so many of those little seemingly insignificant teegage-year things. Like, staying out too late, having girls (or guys, lol) call my house, and just being a normal teen.
You can still do many of the things that you missed as a teen. Being older you have even more opportunity.
I like the fact that life isn't rigid. People don't have to be compartmentalised. Things are not black and white. You may have a teacher in school who is a real hard-ass and strict, but in the night he may be out there doing monkey impressions on a table in a pub- lol.
What i mean to say is just because you have these changes and maybe increases in responsibilities don't deny yourself opportunities or tell yourself that you can't or shouldn't do certain (un-damaging) things, just because it doesn't follow the main stream stereotypical way of thinking. Just be yourself and follow your heart.
I still have a teddy bear for example- and i am 21! and i don't care what people think about that.