This study examines individual growth rates in definitional skill over a period of three to six years, for 68 low-income children. Children were asked to define words once a year at school, from kindergarten (youngest administration at 5;3) through fourth grade (oldest administration at 10;10). A plateau was observed between age nine and ten both for percent formal definitions (characterized by presence of a superordinate) and for the quality of formal definitions. The plateau was lower than the theoretical ceiling for these measures. However, the children appear to have attained ‘adult levels’ of definitional skill: forty-seven fourth-graders (aged 9;1 to 10;10) performed higher, on average, than their own mothers when giving definitions. These results support the notion that definitional skill is related to being part of an academic culture: low-income mothers, whose formal schooling is complete, generally do not give oral definitions to simple nouns as well as do their nine- to ten-year-old children.