My DD started sleeping on her tummy at about 12 weeks old. Once she knew how to roll over, I couldn' stop her from sleeping on her tummy. DD is now 13 mths old and we always still put her to bed on her back, but when we check her she has rolled over on her tummy, she is more comfortable and sleeps better. I was told by MCHN that if they have head control and can move their head up and down and side to side then it is ok because they won't suffocate, if they can't breathe, they will move their head.