I started it when my daughter was just over 2 years old because her school asked me to.  I tried for while by putting her to potty every 30-45 min. but it didn't work.  Then her teacher told me to bring her some underwears and extra pants and no diaper.  It took about 3 days (we did it at school and at home) and she was o.k. although poopoo training took a little bit longer.

Denise