Objective: ADHD is one of the most frequent neurodevelopmental disorders. In addition to clinical assessment, its diagnosis requires the use of validated and reliable behavior questionnaires such as the Conners' Parent Rating Scale-Revised: Short Form (CPRS-R:S). Though various French versions of the CPRS-R:S have been already put to use in clinical practice and research, only a few have undergone a stringent validation process. After a previous validation of the factorial structure of Lausanne French version, we sought here for the analysis of its invariance across sex and age. Method: This validation step was carried out in a rather homogeneous French population of 365 boys and 374 girls from a single school. Two-age classes were considered: children (boys and girls) aged 9 to 11 years (n = 258) and adolescents aged 12 to 15 years (n = 481). Results: Regarding age, dimension Oppositional showed a strong invariance whereas dimensions Hyperactivity and Cognitive problems/Inattention showed a partially strong invariance. Regarding sex, dimensions Oppositional and Hyperactivity showed a partially strong invariance whereas dimension Cognitive problems/Inattention showed a partially weak invariance. The distribution of the CPRS-R:S scores is given by sex and age class. Conclusion: The Lausanne French version of the CPRS-R:S, already validated regarding its factorial structure, internal consistency, and reliability, is here validated regarding its invariance across sex and age. Caution should be taken in using dimension Cognitive problems/Inattention in comparisons between boys and girls.