Well pretty bloody experience we have had here!!
AC320! Its not necessary that they grow, and if they do, usually they grow in the late teen age or early 20's. SOme have none, some 1, 2, 3 or 4. Four for me.
They grow behind your last teeth, and its a REAL hard pain when they are growing. After they are there and the pain is gone, its particularly hard to clean them, which so often results in the rottening of these teeth and if someone doesnot act fast enough they effect the teeth next to them. And as clwar by the Jsc17's post its not a very pleasing experience taking them out, definately because of their odd position and secondly they are perhaps more deeply rooted in the gums.
Didn't quite understood what you said IHadAPheo, were you a heroin adict? Glad if you have recovered now. How hard was it and how do you feel now??