New teeth erupting. Close-up of the mouth of an 7-year-old child, showing eruption of a permanent incisors in the lower jaw. Permanent teeth start to erupt at around age six. They replace the milk teeth (deciduous or primary teeth), which loosen and fall out due to reabsorption of the roots. The full set of permanent teeth is complete by around age 12, with the exception of the wisdom teeth, which usually erupt between 17 and 21.