I'm a 22 year-old body toting around a 13 or 14 (maybe more like 11 or 12, depending on who you talk to
) year-old mind. I always have fun no matter where I am, but within the bounds that are defined for each circumstance. I have the maturity to act appropriately when the time is needed, but that was instilled by my wonderful parents, not age. I can have fun and joke and not worry about what people think. I plan on being a big kid all my life. It keeps life fun.
P.S.- the biggest reason 20 sucks is because you're no longer a teenager. You will feel it, trust me. You'll be doing something or sitting somewhere, and you'll suddenly snap and think to yourself, "im not a teenager anymore...FUCK....."
It's not all bad, though. When you hit 21 (here in the US) you can drink, so that's a cool age... but then you hit 22 and got nothing again. lol don't stress, take this in stride.