The Belgian population is ageing due to demographic changes, so does the workforce of firms active in the country. Such a trend is likely to remain for the foreseeable future. And it will be reinforced by the willingness of public authorities to expand employment among individuals aged 50 or more. But are employers willing to employ older workers? The answer depends to a large extent on the ratio between older workers’ productivity and their cost to employers. To address this question we tap into a unique firm-level panel data set to produce robust evidence on the causal effect of ageing on productivity and labour costs. Unobserved firm fixed-effects and short-term endogeneity of workforce age pose serious estimation challenges, which we try to cope with. Our results indicate a negative productivity differential for older workers ranging from 20 to 40% when compared with prime-age workers, and these productivity differentials are not compensated by lower relative labour costs. Furthermore, the (now dominant) service sector does not seem to offer working conditions that mitigate the negative age/productivity relationship. Finally, older workers in smaller firms (<100 workers) display a larger productivity differential and a productivity that is less aligned on labour costs.