Nine and ten year old tires. It's what you feel comfortable doing. I have heard of new tires that didn't last so years on them not good. Few hundred dollars for new ones and your safety chances are better. Here's a clip[ on aged tires:
http://abcnews.go.com/GMA/video/aged-tires-driving-hazard-4826897
http://abcnews.go.com/GMA/video/aged-tires-driving-hazard-4826897