This tooth is the bottom central incisor. Your child should have 20 primary (baby) teeth by 2 ½ to 3 years of age. The first permanent molars, or 6 year molars, come in around the age of 6 and they erupt behind all of the primary teeth. Your child should lose their first tooth (lower central incisor) around the age of 6-7 years. The average age for a child to lose their last baby tooth is 12 years of age.