I was raised with well water, and I didn't even use fluoride toothpaste until I was around 6 or 7. I had 3 cavities, which they filled, and that was it. I went to the dentist yearly for my cleanings, until I was 45, then went twice a year for cleanings. I think they replaced my fillings twice in my lifetime. The problems I'm having now, are not due to lack of proper care.
The first time I went through chemo, I went to the dentist just prior to treatment, and made sure my mouth was in tip top shape. My dentist warned that sooner, or later I would most likely have issues. It affected my salivary glands, so I don't produce enough saliva. This time, I went and got everything done, but the dentist warned me again, that I would have more issues. I did. It's just the nature of the beast, not a lack of good dental hygiene, or anything genetic.