Understanding the typical timeline can help make sense of why doctors recommend screening and treatment at specific ages, rather than a “wait and watch” approach.

*Birth through childhood: The gene change is present in every cell from birth, but the colon is usually still clear of polyps, or has only a few scattered ones. This is often when genetic testing is offered to children of an affected parent, specifically to work out who has inherited the gene change and needs to begin screening, and who hasn’t and can be spared unnecessary procedures.
Early teenage years (roughly age ten to fifteen): Small polyps typically begin to appear in the colon and rectum, initially few and small.
*Late teens through the twenties: The number of polyps steadily climbs, often reaching into the hundreds or thousands. Individual polyps also begin picking up additional, separate genetic changes over and above the original APC change, which is the same step-by-step process seen in ordinary colon cancer, except here it’s happening in many polyps at once rather than just one.
*By the late thirties, without treatment: The risk of colon cancer becomes very high, close to certain. With so many polyps present, each with its own independent chance of change, it becomes a statistical near-certainty that at least one will progress to cancer. On average, without treatment, colon cancer is diagnosed around age thirty-nine, decades earlier than typical colon cancer in the general population.
*The milder (attenuated) type follows the same underlying process, just later: polyps tend to appear in the twenties or thirties, and while cancer risk is still meaningfully higher than average, it tends to develop around a decade or more later than in the classic type.

This is also why lifelong follow-up matters even after the colon is treated: this condition can affect other parts of the body too. This includes a lifelong, elevated risk of polyps in the upper part of the small intestine (called the duodenum), where doctors use a specific staging system to track how advanced any polyps there are and how often to check; a risk of desmoid tumours, the firm, non-cancerous but locally persistent growths mentioned earlier, occurring in roughly ten to twenty percent of people, often after abdominal surgery; a higher risk of thyroid cancer, particularly in young women; and, rarely, a form of liver tumour that can occur in early childhood, which is why some families are offered simple screening for their infants. Your care team will guide you on exactly which of these apply to your situation.

The reassuring part of this otherwise serious picture is that this predictable timeline is exactly why this condition is considered highly manageable when caught early. A cancer outcome that would otherwise be near-certain becomes largely avoidable through timely monitoring and treatment.