There is a startling statistic that shows colon cancer for older people is dropping, due to routine colonoscopies recommended for those 50+, but is increasing in those that are in their 20's and 30's. It's a more aggressive type too.
All I can recommend is that if one has previously unexplained constipation, thinning stools and/or blood in the stool, get checked right away and insist on a referral for a colonoscopy. My doctor initially thought it was constipation or IBS and it was two months before a referral to gastro.
In 5-10 years I think medicine will start to recommend a colonoscopy at age 30.