A 35-year-old woman is in your office for a consultation for asymptomatic grade 1 internal hemorrhoids, for which you recommended fiber supplementation and conservative measures. At the end of the visit, she also asks when she should get a colonoscopy, because her 40-year-old brother had a recent colonoscopy and was found to have a tubulovillous adenoma; their father was diagnosed with stage II colon cancer at age 49 years. There is no other family history of cancer. The recommendation for her colonoscopy is