Now as he grows, the Chovas Halvavos makes a statement always quoted here, why do teeth hurt when a child begins teething? In order to teach a child the nature of Olam Hazeh; this world is not going to be all fun, and he has to get accustomed to the idea that sometimes you have to bear it, you have to tolerate suffering. That's the introduction to Olam Hazeh. Now once a person learns to tolerate suffering, then he's ready to take it.
As time goes on and if he utilizes the opportunities to protect his teeth properly, if people teach their children to brush their teeth and not go to sleep with a candy clinging between their teeth, letting it rot during the night, then they are going to have good teeth when they grow up, and be good looking too. A girl with nice teeth can more easily get married too, no? Therefore it's important for parents to teach their children to brush their teeth, they'll be happy and healthier, but from neglecting your teeth anything can happen; a person neglects his health anything can happen.
Therefore the teeth are a great blessing, and when the teeth start falling out in old age, it's a sign to get ready for the great transition from this world to the next world.
That's why your hair starts turning white instead of remaining black, to let you know you have to start preparing.
Good Shabbos To All