I have read that an old rooster (5 years old) with a young hen can produce fertile eggs. Also an old hen with a young rooster can produce fertile eggs. Two old chickens will probably not produce fertile eggs. I have found this to be true in my own flocks over the years. I have also found that the fertile eggs from an old hen may not produce a viable chick if incubated. But miracles can happen. I have gotten chicks from 6 or 7 year old hens if they were with a young rooster. 
