I don't think that I had IBS formally, and I wasn't diagnosed, but there were definite signs..
In elementary school I would be scared to go the bathroom in school and other public places and hold it all day long until I got home. Hah... cruel jokes on me now, considering i've hit almost every public bathrooom in town nowadays.
Also, when I was younger I remember sometimes not going for 5 or 6 days at a time, but it never hurt, or made me feel gassy or bloated.
When I turned about 15 though, I would get D occassionally. I remember once having to go on the field hockey bus home from a game, and basically crying the entire way home while I held it. I was formally diagnosed senior year.. or maybe freshman year of college, because by the end of high school I had D a lot more regularly.