In psychological assessment of children, it is pivotal to establish from what age on self-reports can complement or replace informant reports. We introduce a psychometric approach to estimate the minimum age for a child to produce self-report data that is of similar quality as informant data. The approach makes use of statistical validity indicators such as person-fit and long-string indices, and can be readily applied to data commonly collected in psychometric studies of child measures. We evaluate and illustrate the approach, using self-report and informant-report data of the PedsQL, a pediatric health-related quality of life measure, from 651 child–mother pairs. To evaluate the approach, we tested various hypotheses about the validity of the self-report data, using the 퐺푝푛 person-fit index as the validity indicator and the mother informant-data as a benchmark for validity. Results showed that 퐺푝푛 discriminated between self-reports of younger and older children, between self-reports of children that completed the PedsQL alone or with a parent, and between self-reports and informant reports. We conclude that the validity-index approach has good potential for future applications. Future research should further evaluate the approach for different types of questionnaires (e.g., personality inventories) and using different validity indices (e.g., response-bias indices).