An important task for future research, and for the construction of unbiased test batteries, is to investigate whether the gender differences in the PISA and PIAAC surveys are age-specific or ageinvariant. In both cases, educational systems as well as skill surveys will need to be partly restructured. The changes will most likely have to be much more far-reaching if the gender differences are age-invariant. It is important for policy-makers to be aware of this when taking decisions based on these assessments.