Great question! I can hardly wait to read all the answers.........
The youngest bull I ever heard of was about a year old........but it's been a while since I heard that, so I could be mistaken.......hence the "about" part of the statement.
He belonged to some neighbors who were going to sell him as a bull, so put him in with the weaner calves......about 9 months later.........whoops!
He was the only "bull" in the vacinity, and a lot of the calves had his markings, etc. so they decided it must have been him.......