In critical period hypothesis (CPH) research, native speaker (NS) norm groups have often been used to determine whether nonnative speakers (NNSs) were able to score within the NS range of scores. One goal of this article is to investigate what NS samples were used in previous CPH research. The literature review shows that NS control groups tend to be small and highly educated and that detailed background information is usually not provided. Another goal of this article is to investigate how the NS norm group may affect the incidence of nativelike performance by NNSs. To this end, 124 NSs and 118 NNSs of Dutch completed five comprehension tasks and a vocabulary task. On the basis of mean scores and standard deviations, norms were determined for a representative and a nonrepresentative (highly educated) subsample of NSs. Also, separate norms were constructed for the high- and low-frequency items within a task. Exact McNemar tests were used to establish that the incidence of nativelike performance by NNSs was significantly higher if a representative sample norm was used. The results also showed that, insofar as there were effects of frequency, norms based on low-frequency test items tended to be more inclusive. The results imply that the selection of NSs in CPH research deserves more consideration than it has received in the past; they also suggest that NS ceiling performance is potentially useful in determining nativelike performance.