In answer to your first question, no you do not have to show in trios if you are showing at an APA/ABA sanctioned show, you may if you desire but must specifically mark it on the entry form and they will be judged and receive points accordingly. County and state fair rules will vary and may require showing in trios.
Your second question can be easily answered in that the older the bird the better the chances of it competing well. Young pullets and cockerels will lose to older birds that are of equal quality according to the Standard. Always try to hatch your chicks as early as possible in the year giving them adequate time to mature before the fall show season begins. I usually begin setting and hatching in late December/January.
Best wishes in your poultry showing endeavor.
Bo