This can be done by 'Controlling for age' in statistics. If your cohort gets older, you get more prevalence of old age diseases of course. But by isolating by sufficiently small age groups (assuming the age distribution is approximately constant, or flat, within a group[1]), we can tell if the rate per given age is increasing or not. In this case simple multivariate statistics to judge if the changes are significant.

[1] The variation in distribution among age groups can be controlled by more sophisticated methods, but it may be sufficient to make the groups small and multivariate analysis (although results might be weakened by assumptions of independent errors if groups are too small).