Even though I've always loved the song "I'll won't grow up" from Peter Pan, I'm actually rather glad to have done so. Despite the annoyances of taxes and insurance and things, there are some definite up-sides.
Being allowed to cross the street without having to hold anyone's hand
If you want to order only appetizers, you can
Getting to set your own bedtime
It's not weird anymore if none of your friends are actually your age
(I was always the odd one out because I thought the adult conversations were more interesting than the kids my age)
No one can make you eat vegetables you don't like
Batman is still the coolest superhero