The short answer... any time you want.
The long answer...
The earlier you start, the fewer "readiness" signs your child shows, the longer you will be training. No matter what age you start, a child will fully take over his "potty duties" - getting himself to the potty when he needs to go and using it with little to no help - at about the same age. So, you can start shortly after birth and be done at 38 months or you can start at 36 months and be done at 38 months. It all depends on your child.
It's your choice: fewer diapers, more effort, longer time, bragging rights of "we've started training" OR more diapers, less work, shorter time, bragging rights of "we're working with our child, not forcing him". Both have pros and cons. Your choose what's right for you.
at 10:19 PM on Feb. 18, 2009