I have an 18yo bill for a medical lab, and I still get my regular bloodwork there... It gets brought up now and then on current visits, and I just say, "I'm not paying that bill."

As to the why, it's because I switched jobs and got my regular labs done and apparently was between coverage that week... they tried to charge me a large multiple of what they charged insurance for the same labs, so I refused to pay.

I also had several bills that were never paid from a week long hospital stay in my mid 30's... Most of them got paid if they'd agree to reasonable payments or settle for lower amounts when I had the money (tax return time, bonus, side work). After 7 years, they were all off my credit and I stopped bothering...