Baby teeth usually come out on their on, but when they get knocked out or have to be removed due to dental disease they need to be replaced. Some times the adult teeth do not develop and the baby teeth remain into adulthood, but since they were not meant to last that long they may come out and need to be replaced. The treatment required to replace them depends on the age of the child or adult and the position of the tooth.
For young children if the missing tooth is in the back, it is important to “keep the space” the tooth occupied till the permanent tooth erupts. This is done with space maintainers which are made of wire and prevent the neighboring teeth from migrating or overerupting. If the missing baby tooth is in the front, there is less risk for the adjacent teeth to shift. But if the child is 3 years old, when the teeth came out, he will have to wait till he is 7 for the new ones to come through, leaving the child without front teeth for 4 years. Since children are asked by their peers about the missing teeth, replacing the missing teeth can be a good idea. The solution for replacing them are small dentures or if possible fixed appliances.
For adults, the presence of a baby tooth in their mouths, can come as a surprise when one of these teeth looses the bone support and comes out leaving a gap. The best solutions for these gaps is placing and implant in place of the baby tooth, providing function and a good aesthetic outcome, or an orthodontic treatment to close the gap. Other solutions are placing a denture or a bridge but these can have detrimental effects on other teeth.