[pouts] I don't want more responsibility, I want chocolate milk! [/pouts] 
You can still have chocolate milk (although I can't). Hell, when you get older, you can have more chocolate milk, as you're making some of the rules. The responsibility aspect comes in when you realize you have to fend for yourself. You have to get a job and earn money to pay for a place to life and buy the things you need to survive, I.E. Food, Clothing, Gundam Models, Mango Lemonade, etc. You no longer have the Magic Laundry Box, in which you put clothes which disappear suddenly and then are put on your bed, all nicely folded, by the Magic Laundry Fairy. No longer is dinner prepared for you. Free time becomes less and less available to you.