I never had an imaginary friend (that I remember), but one of my best friends growing up did. I had a younger sister, but she was an only child. She had these friends until we were at least 12 or so. She never mentioned them to anyone but me as far as I know, but she fully believed that they were real on some level.
She told me that I could make up some imaginary friends to play with hers, and I did, but they were never real to me. One of them was a boy a bit older than her, and the other was actually a fairy. Even though she called them her imaginary friends, she seemed to believe they were real in some way long past the age that kids normally believe in such things.