Genetics has to play a role. I'm 30 and I've never had a single cavity. I was a slob for the first ~18 years of my life and barely brushed at all because I knew I didn't really need to. Now I brush once a day, but never before bed, only before leaving the house. I've flossed at most ten times in my life and pay no particular heed to my diet (although I am not unhealthy or overweight). Meanwhile my sisters and parents are far more attentive and all of them have had multiple cavities. We all go to the same dentist, and have done for 30+ years.