<prompt> 21st tooth erupts at:-6 years8 years10 years12 years  <response>: Eruption is defined as the superior pa of crown of the tooth appearing level with the surface of the alveolar bone.Temporary dentition: total 20 number of teeth, stas at 6 months till 2 years.Permanent dentition: total 32 number of teeth, stas at 6 years completed by 17 - 25 years.1st to erupt is molar 1 M1 and last to erupt is molar 3 M3.21st teeth refers to 1st permanent tooth, which erupts at 6 years. 