when to potty train

I think that I started trying before my son turned two. I was in a hurry to stop changing those diapers. lol. But he went when he was ready and now he is three and a half and he has been completley potty trained for at least over a year. If not longer. But all kids are different. I have a friend who's son is a couple of months younger than my son and still in diapers. Good Luck!