With chrons diseace being inherherited how could you go 40 years wih no stomach problems why would syptoms not startearlier?

Crohn's onset: Most people with Crohn's disease will experience symptoms between 15 and 30 years of age, but onset can occur at any age. Mice with mutations in a Crohn's disease gene require a virus to trigger symptoms, according to a new study. The finding may give insight into why some people with genetic risk variants for complex diseases remain unaffected.' -NIH 2010