It might not be an environmental cause - if people are genetically more susceptible to those cancers when aged between 20s and their 40s then a lot of people in

It might not be an environmental cause - if people are genetically more susceptible to those cancers when aged between 20s and their 40s then a lot of people in the older age ranges would have already died from them before the year 2000 and so wouldn't be caught in their data. If they want to confirm it is an increase then they could repeat their analysis on a set of data covering years previous to 2000.