My daughter bought hers in August this year so they will be barely under a year old by next year's July fair.  (she is a thinker that one!)  Then she can show her ones she showed this year as over a year old.  That way she can maximize her trip and get to show as many birds as she can.  
The poultry judge told us anything under 9 months is not fully feathered enough to judge properly.  So that can give you a guideline!   Of course the meat pen birds normally are ready for butcher by 6-8 weeks so use that for a judgement.  Also in our case, we have our fair then 4 weeks later is the state fair for our meat birds we have to have 2 batches or the ones we show at county would be to large by state.