I think a bunch of the previous posters hit the nail on the head: the freedom of adulthood is awesome. Yeah, it's your responsibility to support yourself, but once you've got that out of the way, you basically decide what's up.
Though I'm not noticing a ton of people mentioning the other thing that sucks about getting older: Aging.
Getting older for me has meant sudden, terrible lactose intolerance, a knee that's gone clicky from ten years reaching into lowboys, and a back that absolutely needs my pillowtop mattress. I cannot crash over at a friend's house on their couch. I'm freakin' old now.
Do I still get my comic books, spend too much money on Steam, and watch cartoons? Hell yes. But I'm also old, so I absolutely have to get out on the bike or go to the gym or my body just starts to fall apart.
As a teenager, and then in college, I could bounce back so easily. Minor sprain? Fuck it, walk it off. Stayed up too late partying? Slap some concealer on the shadows under your eyes and get on out there. Or call one of your classmates and get the notes from class.
But as an adult I find myself having to make the choice of: "No. No whiskey. Yes, Dad gets whiskey, but you have to work tomorrow, so you get a pint of beer with dinner. One pint. Seriously."
Or: "As much as I would like a huge plate of fries to go with a bacon-burger, I should probably do the soup and salad."
Or even: "As much as I would like to do the soup and salad because I am damn tired and don't want to cook anymore today, the wallet is a little thin after Christmas, so it's actually going to be a can of soup and a bag of carrots. Also, no beer."
Or worse: "I'm very tired and I'm pretty sure I ate a full meal tasting everything I cooked at work today. Maybe I can just fall asleep."
How about this one? "Oh man, I would love to go see a midnight premiere of Awesome Movie... but I have a morning shift. Can we catch a Sunday matinee?" Maybe your parents didn't let you go when you were a kid, but at least you could blame them. You have nobody to blame for this boring responsibility but yourself.
Or good ol': "Yes, I'm working on my birthday, but it's on a Thursday this year so all my friends are working, too, and there wouldn't be anyone to party with anyway."
Basically, adulthood comes with all the freedom to fuck yourself and your life up as much as you want. You can choose to never clean your room or live entirely off of gummy worms, and you can go to bed whenever you want to. But eventually you're wading knee-deep through trash, you haven't pooped in forty-eight hours, and you slept through your alarm and got fired. But at least it's your choice.