Totally depends on the kid. My first was 3 1/2, my second goes on the potty once in a while and she's 1 1/2. Watch for signs that they are interested and can communicate the need.
that is absolutely individual to each child. my son was 3 1/2, my daughter was 2 years 9 months. preschool was the motivating factor for me to train them. both trained lickety split!