Sex differences in the foramen magnum region of the cranial base have been identified with varying rates of success. Recent publications demonstrate a continuing strong interest in metric analysis of the foramen magnum region for sex determination despite the generally low expression of cranial base sexual dimorphism. It is important to identify possible age effects on skull base morphometric variables as most reported discriminant analyses use pooled-age samples without assessing the influence of aging on sexual dimorphism. This study examined 135 adult cranial bases (69 males and 66 females) from the St. Bride's documented skeletal collection in London. Traditional craniometric measurements were recorded and the effect of age on sexual dimorphism of this anatomical region was tested using a variety of statistical analyses including MANOVA and discriminant function analysis. Age-dependent discriminant functions for <50 and >50 years of age were developed and compared. The cross-validated results showed that the <50 years function determined the sex of 69.1% correctly while the >50 years function achieved 81.3% correct predictions. However, the high sex biases of these functions (14.4% and -17.5%) severely limit their practical application. A pooled-age discriminant function permitted 71.9% correct prediction with a sex bias of only -1.7%. The statistical analyses also showed no significant age effect on any of the variables, suggesting that a separation by age is not necessary for the development of sex determination methods.