That's one of my favourite questions.
I find it amusing that most people's first response usually is: What do you mean, I am a grown up. But I grew up with this question being used at home for fun. My mom would use it to change the world by saying "When I grow up I'm going to be the King and then noone will be <fill in whatever injustice that comes to mind.>"
So all of you lovely people: What are you going to be when you grow up?
Currently I'm sort of inbetween (a) opening an übercool hostel out in nowhere with lots of things to do at very decent prices so people would stay for a very long time, or (b) use my abilities to do some good not directly linked to this ROI-infested business I'm currently in.