We surveyed a group of 56 children (aged 2.3–11.3 years) with complete deciduous or mixed dentition who received brushing instruction three times a year at four-month intervals for 3–9 years. The labial, buccal, lingual, and occlusal surfaces of the deciduous dentition were divided into 20 areas, and those of the mixed or permanent dentition were divided into 28 areas. The age at which children were able to consistently reach all these areas was designated as the age at which children were competent enough to brush their own teeth (“the complete practice age”). Until children reach this stage, parents should brush children's teeth. In this study, the complete practice age was frequently 8–9 years in children who received brushing instruction from before the age of 8 years. However, in children who commenced brushing instruction after the age of 8 years, the complete practice age varied, and an appropriate parental brushing period was difficult to determine. However, no children reached the complete practice age in less than a year after instruction commenced, irrespective of the age that brushing instruction began. Therefore, we suggest that brushing by parents is necessary for at least a year after brushing instruction begins, even in children who begin receiving brushing instruction after the age of 8 years.