18mo to 2 yrs. Developmentally a child is ready to begin understanding the concept of time out between 18 months and 2 years. Make sure you use time outs immediately after the unwanted behavior happens, otherwise they are not effective. The length of the time out should be no more than the child's age in years. 2 yr old = 2 min time out. Sometimes it helps to set a timer to show the child how long it will be.
Agreed 18months. I agree that the most appropriate time to begin using time outs is around 18-24 months. Children may not completely understand why you are doing the time out and trying to explain complex ideas to them is ineffective. But toddlers can clearly learn from cause and effect and follow clear and consistent patterns. Doing this behavior leads to this undesired outcome - i.e. Time outs.