I don’t think your interpretation is correct. I think that comparison was of data for the two cohorts when they were the same age.

The study, yet to be peer-reviewed, found 28,265 cases of early-onset bowel cancer over the 30 years studied. An estimated 4,347 additional cases were attributable to the rising rates of the disease.

Early-onset cases, however, are increasing by up to 8% per year, the study found.