I've shared this xkcd a million times. Never gets old.
When your 10:30 bedtime rolls around, you can say, "Nah... Time for another episode of Arrested Development."
You worry about the little fuzzballs/poopers when you go away, but it's oddly satisfying that you have a little someone to worry about.
Mama always said you could be whatever you want when you grow up. Turns out you're not limited to one choice! You can be an astronaut, a firefighter, and a teacher! (Though why would you ever quit being an astronaut?)
Learning doesn't stop after school is done. (Check out the Richat Structure!)
Dancing to your favorite songs for half an hour is totally exercise. There are lots of fun ways to exercise as an adult...
It's harder to make new friends as an adult, but you can pick your friends' traits, to a degree. Want to make friends with an artist? Take an art class. A marine biologist? Talk to the people who run the local aquarium.
You can develop new hobbies whenever, and no one will tell you you can't. Unless you take up throat singing.
Popcorn and ice cream for dinner!
Best of all... you get to do everything you did as a kid. Dress up, make believe, play outside all day, make mud pies, build a blanket fort - the possibilities are endless.