There is no definitive answer as to when you should start potty training your children.
Each child is different and has different needs. Your first child may be ready at 2 but your second child may not be ready until 3 years old.
A recent study published in the Journal of Pediatric Urology suggests that potty training is best when the child is between 2 and 3 years old. More precisely, the ideal window is between 27 and 32 months. Waiting any longer than 32 months could make your child develop “urge incontinence,” or daytime wetting and bed-wetting.
Others disagree that age should be the deciding factor in beginning potty training.
Peter Stavinoha, a clinical neuropsychologist at Children’s Medical Center of Dallas, says that start potty training when the child is ready, not when you’re ready.
Many parents approach potty training as something over which they have total control…Parents are a big contributor to their child’s development, but they don’t really control it. Parents are there to facilitate, to guide, to reinforce and to praise…
…not to control!
Potty Training Best Between Ages 2 and 3 [Yahoo!]

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Of course, every child is different and there is no one correct age, but it is interesting to note that the average potty training age in America has crept up to 34 months from 18 months in the 1940′s.