I agree with Neffer. She does have to find out, and it is best she does from you, rather than your ex or classmates. Or in a position where she might need something medically and she has to find out you can't provide it because you are not a biological parent.
The advantage I think is that at her age, she doesn't know a father as something biological. She knows Daddy as the person who loves her and takes care of her. So even if you explain that you came into her life after she was born, she has always been your daughter and baby in your heart.
