Teenagers

Why do teens think they are smarter than everyone else and treat parents/grandparents as if we were never teens and don't have a clue what they go through?
My 14 year old granddaughter is doing this....I will date whoever I want... why are you always against me and what I want to do? Why can't they understand it is because we have been there is why we fight to keep them from going through the same hurts and pains?