Aim: Identifying factors that influence children's and adolescents’ participation in physical activity (PA), as well as their physical fitness (PF), is essential for the development of effective intervention strategies. The aim of this study was to investigate and compare influential socio-demographic factors that affect PA such as socio-economic status (SES), rural–urban differences, immigration and age, as well as the effects of age and PA on PF as differentiated by gender. Subjects and Methods: German children between 6 and 9 years and adolescents between 10 and 17 years of age (n = 2574) participated in the representative, nationwide, cross-sectional ‘Motorik’-Module study between 2003 and 2006. Results: Results revealed that immigrant children and children with a lower SES background were less physically active and that this inactivity subsequently resulted in lower levels of PF as compared with non-immigrant children and children with a higher SES background. PA was further positively associated with age for children. All of these three socio-demographic factors were comparably meaningful for PA. In adolescents, the only PA-relevant socio-demographic parameter was SES with lower PA again resulting in lower PF levels observed in adolescents from lower income families. PF in childhood as well as adolescence was mostly positively affected by age, followed by PA, except for in female adolescents for whom PA and age were nearly equally significant. Conclusions: An intervention aiming to improve PA levels, and consequently PF levels, must, with respect to age and gender, refer to SES, as well as immigration background, but not to rural–urban differences. [ABSTRACT FROM PUBLISHER]