What causes teeth to become grey?

Seveal things: In a young person, teeth can be stained by the use of tetracycline (an antibiotic). Once the teeth are formed, grey staining might be related to wear of the teetn which has caused a loss of enamel...Allowing staining substgances to seep into the dentin which underlies the enamel. Look for mineral sources in water. Teeth can be whitened by a dentist to return to a white appearance.

Several factors: Tetracycline antibiotic can cause graying of the teeth, also some vitamins with high iron content and several foods like berries, wine.

Agreed.: Grey color can be caused by anything from medication your mother took during pregnancy to fillings you had done years ago to something your eating now. Staining can occur during development (before you ever see the tooth come through the gums) or anytime after that (unless you only eat white food). Talk with your dentist about things to avoid.