FWIW, I didn't start having problems with my teeth until I was in my 20's, and that was after years of diligent brushing and flossing (I saw the problems my mother had and didn't want to deal with them myself!). At that point I started getting cavities, then moved on to root canals and crowns in my 30's. Genetics plays a big role in what your teeth do.

I'm not sure what I'd tell my younger self. I would absolutely tell my 4 and 5 year old self that pink looks just fine on redheads (my mother dressed me in mustard yellow because redheads aren't supposed to wear pink, you know) and my teenage self that my sister is full of baloney and I'd make a fine architect (sister convinced me not to pursue architecture because the math was too difficult. She was two years older than me and I was in her math classes throughout high school. Why on earth did I believe her when all evidence pointed to her just being jealous?). I'd probably also tell my college age self that I might regret indulging in the free flowing (cheap) champagne at the Christmas dance. I'm pretty sure my college age self wouldn't listen. However, she might take one look at her future self and decide to lay off the Belgian waffles for breakfast!

Sarah