There is much variation between bulls, and it involves more than just testicle size. We leased CCS Marc Drive to ABS several years ago when he was 9 months old. He was sent to ABS in Wisconsin immediately as they had a truck going there. They did not think he would produce semen for a few months but to everyone's surprise they were freezing high quality semen before he was 10 months old. I have seen other bulls who do not have mature enough semen to freeze at 15 months of age. Hopefully your bull is one that will freeze at a young age. About the only way I can think of to know for sure is to have him collected, and see how he does.