What do you want to be when you grow up?

What do you want to be when you grow up?  This is easily the most common question adults ask children.  It is always fun hearing the answers, and wondering if that little boy might really become an astronaut or if that little girl will be a doctor.  Now, my little lady, she wants to be a rainbow ballerina princess.  I'm cool with that!  I remember people asking me this question at various ages in life.  My answer was usually that I wanted to be a doctor, but mostly because it seemed like the right thing to say.  The truth of the matter is I am definitely not a doctor, and I still don't feel like I am doing what I want to be doing.  By what age are you supposed to have it all figured out?  And HOW do you figure it out?