I am an old person (70) and have been fortunate to have had several superb dentists in my life. ALL of them shared the belief that a person's dental health is principally genetic, dependent on the chemical composition in the mouth which is not much influenced by either food choices or dental hygiene.
I don't know whether this is true or not but my brother is a good example of this belief. Although he improved his diet in his old age, as a young person, he not only ate the SAD but loved sweets, starches, etc. As a child, he was very careless about tooth brushing, avoiding it as much as possible, despite my mother's nagging. He never had a cavity in his life nor any gum disease, etc.
My last dental visit also illustrates this. I've been low carb for years, eating primal for the past 2 years. I rarely touch sugar, no grains, and no dairy. I brush and floss regularly. My dentist commented that I had so much tarter on my teeth that he assumed I had not had a cleaning for a year until he checked the record and realized it was only 6 months. When I was shocked, he repeated what I've heard from other dentists--the chemistry of my mouth is mainly a genetic issue, not much influenced by my behavior.