Well I have an answer to this question and it is YES, they can be fertile at quite a young age. I just recently witnessed my 13 week old cockerel mounting my laying hen. I thought, "There is no way he is fertile yet, he is only 13 weeks old!" I came here and read this thread and thought, "See, he couldn't be, but he sure has the gymnastics down." Then over the next few days I watched him try to mount the other 13 week old pullets but they would just run from him. My laying hen however was older and knew what she was suppose to do so she would squat and let him jump on. After he was done (which sadly doesn't take long) she would get up, fluff herself out then checked her rear end (?), interesting.
I read somewhere that if he is able to manage the moves then it means he is fertile. To test this theory I decided to collect her eggs up for the next 4 days. I waited 2 days, then began collecting. Then I cracked them open and all 4 eggs had the definite 'bullseye' that indicates a fertile egg. Since I already have my incubator running I put one of her eggs into incubator and I will candle it after a week to see if anything is happening inside.
So the answer is Yes, a cockerel can be fertile at 13 weeks old and he can successfully mate with a hen.
BTW, I know the exact hatch date of my cockerel so there is no discrepancy on his age.
