Whatever age you want. But until she shows an interest and can stay dry for 2 hours or more at a time, you are wasting your time. It doesn't have to be dry overnight or even dry during a nap - just dry for 2 hours or more at a time. That let's you know that she is learning how to control her bladder. You don't even have to wait until she can (or will) tell you. Most toddlers quickly STOP telling mom that they have to go or lie when asked simply because playing is more fun. Telling you she has to go also means putting down the toys.... no fun for a toddler. That's why once you start you should not ask her if she has to go or wait for her to tell you. You simply tell her it is time to try.