My molars all have large fillings. Two of my back teeth have eventually shattered because the fillings were so big, so now I have crowns on those shattered teeth. I have 3 sons (11, 15, & 17), and they've eaten a lot of junk over the years, and only one of them has a small filling, and he just got it this past year. I regularly brushed my teeth as a kid, and I ate a lot less sugar than my kids.

I think that it has a lot to do with genetics, because my mom and I both have pretty soft enamel on our teeth, and we've both a lot of fillings. My wife, however, has very few fillings, and I think that my boys have (thankfully) taken after their mom in that area.