The current paper explores the impact of age and sex on adult cranial shape and size variation in the documented collections from St Bride's and Spitalfields. The scope is to assess the extent to which it is valid to pool age and sex groups in inter‐population comparisons that use cranial data. For this purpose, age and sex differences in cranial shape and size were explored using multivariate analysis of variance, Discriminant analysis and Mann–Whitney tests. The results suggest a clear change in cranial shape with increasing age; however, this change is not statistically significant. Therefore, it is justifiable to pool different age groups in bioarchaeological analyses. Increasing age also has a minimal impact on cranial size among females, whereas among males, its impact is small but statistically significant. Finally, cranial shape and size are significantly different between males and females, irrespective of their age. This dimorphism can be used for the assessment of sex, although attention should be given to over‐classification problems when using discriminant analysis. Copyright © 2012 John Wiley & Sons, Ltd.