In Arkansas, as far as heifers and bulls go, you should be able to show them at any age up to 2 year. There may be a few exceptions to allow you to show them up to 30 months. It really doesn't matter how young they are. Of course, you wouldn't want to show a baby. I would say they would need to be at least 4 months old unless they are part of a cow/calf pair.
Check out the arkansas state fair website.