The story I've always heard about this is that as you grow older, your immune system becomes less active in defending your teeth, and so you have to correspondi

The story I've always heard about this is that as you grow older, your immune system becomes less active in defending your teeth, and so you have to correspondingly brush more often to maintain teeth health. So if you're brushing the same amount as you were as a teenager, you will get more cavities. I don't have a citation though.