All stories begin with once upon a time, or they should. So here:
Once upon a time there was a girl who was afraid to grow up. She was afraid that growing up meant settling (down and for) and giving up her days of dress up and make believe. It was like she thought that one day someone would come and say, "you're a grown up now; give us your tutus and your glitter and your imagination."
So she made a promise to herself. To keep living in dreamland and dancing in public and painting each nail a different color. To be Peter Pan not Wendy Bird. To never grow up.
And she hasn't yet.