Children's teeth begin forming before birth. The first baby teeth to erupt through the gums are the lower central incisors, and this can occur as early as 4 months of age. Although all 20 primary teeth usually appear by age 3, the pace and order of the eruption of these teeth varies.
Permanent teeth begin appearing around age 6, starting with the first molars and lower central incisors. At the age of 8, you can generally expect the bottom 4 primary teeth (lower central and lateral incisors) and the top 4 primary teeth (upper central and lateral incisors) to be gone and permanent teeth to have taken their place. There is a one to two year break from ages 8-10 and then the rest of the permanent teeth will start to come in. This process continues until approximately age 21 with the wisdom teeth.
Adults have 28 permanent teeth, or up to 32 including the third molars (or wisdom teeth).