The age ruminant animals come to heat largely depends on their management. However, goat and sheep are supposed to come on heat normally after six months while cow should do same after a year.
It is usually good to mate them during the second heat because by then, they would have developed both physically and sexually. Feeding is a very important factor in determining when they will come on heat. So in order for them to reach sexual maturity on time, you as a ruminant farmer must make sure that they are well fed.
See also: Recommended Number of Ruminant Animals to start your Farm with