We study the effect of the National Minimum Wage (NMW) on the work force age composition within the low paying sectors of the British economy. Our interest is in the degree of substitutability between labour inputs (young and old employees). We find evidence that both the introduction and the regular upratings of the NMW have a significant effect on the observed changes to the relative wages and tothe relative wage bills but not to relative employment. We estimatethe elasticity of substitution, between “young” age groups and older workers (55+) to be zero, while that of “prime” (22+) age and older workers, to be around 0.79. Our estimates therefore imply significant complementarity between younger and old employees.