I was curious about this too and found some information about it. The doctor's on the one site said that it matters your child. Some babies it is good to start them early because of ear infections and congestion problems because it will raise them to a better sleeping position. Another thing they pointed out was if they are sleeping in a toddler bed that it would be a good time to introduce them to a pillow or if a baby likes to climb out of their crib because most will through out stuff before climbing out and it is something soft to land on. I guess it is really just up to a mother if they think they're child is ready.
I don't have my daughter sleeping with one yet and she's 16 months old.