I have had discolored teeth since I was an infant.  I am now 25 years old.  We could never precisely pinpoint the cause or find a cure (other than attempting to take off layers of my teeth and replace it with some finish-I didn't bother).  I was on
milk-free formula as a baby because of a lactose intolerence which has come and gone with varying degrees of intensity through the years.  It would be interesting if there was a connection.
Catharine