Your baby's first baby tooth will errupt at approximately 6-8 months of age. By three, the entire primary dentition will be completed.
At approximately 6 years of age, the first permanent molars errupt behind the posterior primary molars. At the same time, the lower permanent central incisors errupt. Very often, a lower permanent icisor errupts behind the primary one and for some time both teeth are in the mouth. But the tongue slowly pushes the permant incisor forward and the primary one falls out.
The primary dentition is very important not only for chewing, but also for gouding the erruption of permanent teeth in the correct position.