Potty training when they are ready

Readiness is a set of signs, not an age: staying dry for longer stretches, noticing when they have gone, interest in the toilet, and being able to follow a simple instruction and manage clothing. Starting before those appear usually means a longer process, not an earlier finish. Expect accidents and treat them as uneventful. Night dryness follows day dryness, sometimes by a long way, and is not something that can be trained. Pressure and comparison with other children are the two things that make it harder.