As soon as he's fertile, but they don't recommend fertility checking an Angus bull before 12 months of age. We fertility check and sell most of our bulls at 13-14 months old and have no problem with them being fertile. Angus tend to be agressive breeders. If you're using a young Angus bull, watch his body condition closely while he's breeding. They'll sometimes be more interested in breeding than eating and lose body condition.
>Although Angus bulls become fertile around 1 year of age, it is a good idea to hold them back until about 18 months before introducing them to much more that a few cows. when presented with the opportunity to breed 15 or 20 cows, they often will suffer severe body condition loss, thus, adversely affecting their libido. I introduce my 18 month old bulls to 15 cows their first season and then on up to 30 cows when they are 30 months.