We started when she turned 2.
However, she still has the odd accident at night.
I had the opportunity to try GoodNites Disposable Bed Mats from ChickAdvisor.
It is my first time using bed mats, and let me tell you - this bed mat is great.
They are so easy to use, and are good quality bed mats, so you know they won't fall apart if she sleeps on them or soils them.
If you are unsure, there are simple instructions written on the packaging.
They are large enough to cover a big portion of the bed, and they have adhesives to prevent sliding and moving around when she sleeps at night.
The adhesive has a good grip so you dont need to worry it peeling off.
It is comfortable so she can sleep peacefully throughout the night.
I have seen these at Walmart for under $14 for 9 bed mats.
Personally I wouldn't purchase them because I found them to be more expensive than her pull ups.
However, I would recommend these to other parents who does not mind the price.