A bull will often reach puberty by the time he's around 12 months of age (some a little sooner, if the herd's a good fertile herd), and by this time, as a Yearling Bull, he is ready to be used on heifers or sold to another producer's farm or ranch to be used on the heifers and cows there.ost bulls should be showing libido by the time they are 12 months old. However, all this depends on the breed and each individual bull. Some breeds that are later maturing (typically Continentals), like Charolais, Simmental, Limousin or Salers, won't start showing puberty until they're around 12 to 16 months of age.