Autopsy studies show that 1.7% of average-risk people in 20s have an adenoma, so why don't they recommend screening for bowel cancer before age 50?

No value: The only value in removing polyps is the rare one that has genetic changes to become cancer. About 90% don't progress. Survival hasn't changed with long history of polypectomy. In those that do change it takes years for the transformation and for the other lesions, they don't show up as a bowel malignancy until one is usually in their 50's.