Rolling over. Once a baby can roll over well, both directions, between his stomach and his back - this usually occurs by about age 6 months. Before this is true, babies should sleep only on their back to help prevent sids.
Varies. I don't put them down on their bellies until after a year of age. After they begin to turn over on their own around 4-5 mo the stastical risk of crib death has declined & I don't wake them to re-position them.