My kids are upset I never had colonoscopy or mammogram. Healthy and 88. Why should I bother now?

Screening. If you are healthy and planning on remaining so, wouldn't it still be advantageous to detect cancers when they are early, and potentially more curable?
To stay healthy. These screening tests help to pick up problems in the breast and colon before they become serious to prevent having major procedures. If you are healthy, being tested is a great way to remain that way.
Judgement call. Its a judgment call if at 88 you need any of that stuff. Congrats on getting to 88 without breast cancer or colons ca! but for every lucky person like yourself, there are several who are unlucky who never get tested and it's too late :(.