The age of a sheep is generally determined by the teeth in its lower jaw (sheep have no teeth in their upper jaw). Lambs have four pairs of incisors that eventually fall out -- like the "baby teeth" in humans -- and make way for permanent incisors. Each pair of permanent teeth is roughly equal to one year of age, so a yearling willl have one pair, a two-year-old will have four permanent teeth, a three-year-old will have six permanent teetch, and by four years a sheep will have a complete set. "The grinding action of chewing wears sheep's teeth," Storey's Barn Guide explains. "As the incisors wear down, the amount of tooth below the gum line is gradually pushed out to help compensate for the wear. By seven or eight years of age, most elderly sheep are "gummers," having lost all of their front teeth.