Cancer registries in most high-income countries show rising incidence under 50 for several sites since the 1990s, with colorectal cancer the most consistent and the largest. The pattern follows birth cohort — each generation born after about 1960 has higher rates at a given age than the one before — which is the signature of an exposure rather than of a change in detection. But detection is part of the story: thyroid cancer's rise is almost entirely diagnostic, and the lowering of colorectal screening ages in the US complicates the recent years. The strongest argument that the colorectal rise is real is that mortality under 50 has also risen in some countries, which detection alone does not produce. The candidate causes — diet, obesity, antibiotic exposure, the microbiome — are plausible and unproven; no cohort has yet tied an exposure to the cohort effect with any precision. The evidence is therefore mixed: real for some sites, artefactual for others, and unexplained where it is real.