What defines a teenager?

Is it age or attitude that defines a teenager? If it is age, then Rosie, at age four, is clearly not a teenager. However, if it is attitude, we are so there. This morning she couldn't find the right leggings for preschool and she immediately accused me of hiding them from her. She went running through the house screaming "Where are they, I know you hid them from me". Then to top the morning off, after I finished doing her hair, she looked in the mirror and said, "Where did you learn how to do hair, the UGLY hair salon" If this is what she is like at 4 what will 14 be like? Maybe I shouldn't think about it.