21st tooth erupts at:-

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.