To monitor time trends of obesity in children, data from the school-entry screening programs (SEU) are widely used. By using data from the SEU in North Rhine-Westphalia (NRW), Germany, from 1996 to 2008, it was investigated, if age distribution is changing over time and if changes influence the overall proportion of obese children in NRW. In addition, the time trend of obesity in three age groups of children before school entry is shown. Children who were examined during SEU are becoming younger (1996: 75.3 months; 2008: 71.3 months, on average). The observed stagnation of obesity in NRW in recent years also persists when the influence of different age patterns are taken into account. Younger children are less obese than older ones. A sustained increase in the proportion of younger children during SEU might influence the overall proportion of obese children in the future. Thus, it should be discussed which procedures should be used to increase the quality of the indicator that is used for public health reporting and is generated by data from SEU.