My sister had an imaginary friend. It was a cat named Sara...a boy cat. She was obsessed with that cat for years. When she turned six, my mom told her that Sara went to visit our aunt in California. She asked about her for a week or two after that, but finally let go. My kids haven't had one so far, but my youngest is only 2, so maybe.
My 6 year old has a family of imaginary friends. It started out out as only Mr. Ghost he took a year vacation and came back with about 5 family members including Baby Lulu. They travel to places my dd is learning about in history and comes back and tells her about it. They never leave for long anymore.