We investigated sex and age group differences in attitudes to oral health among school-age children using an Oral Self-Care Appraisal (OSCA) to systematically determine changes in oral health knowledge, attitude, and behavior across developmental stages. The subjects (n = 1584) were stratified after random sampling so that each school grade analyzed contained 88 boys and 88 girls. Factor analysis was undertaken to identify a set of underlying factors, with 10 factors considered in the cross-sectional study. Our results showed that the factors toothbrushing, persistence, and sociability were more predominant in primary school children than in junior high and senior high school students. Furthermore, postponement of visiting the dentist and resignation to one's own dental cavities became more predominant in proportion to the level of school education. Girls had significantly higher scores than boys for desire to improve oral care, dental anxiety, dependency on snacks, toothbrushing, concern over number of cavities, and sociability. Together, the results indicated that the oral health care behavior of girls was better than that of boys, and that the tendency to postpone visiting the dentist and resignation to one's own dental cavities increased markedly with age.