I've had loose stools for the past 15 years on a regular basis. Is there anything to worry about if I have a colonoscopy every five years?

Needs evaluation. Hopefully you have told the GI doctor who does your colonoscopies about this. If not, do so. This is not normal and should be investigated. Hope this is nothing serious. Good luck, .
Probably not. There are established guidelines for screening colonscopies, including 3-5 years intervals if no polyps are found. However, the loose stools may not be related to the colon, but the small bowel. I would recommend dietary modification, like a gluten free diet, if you want to address the loose stools.