1 years. Studies show that babies who are put to sleep on their backs have lower rates of sudden infant death syndrome (sids). While it's true that your crawling 10 month old will likely roll around in his sleep, the official recommendation is that children under the age of 1 year should be put to sleep on their backs.
Able to lift head. The official position of doctors is to always put babies on their backs. But realistically, some babies sleep better on their side or belly. When they are able to lift their head up off the bed and look the other direction, or when they are rolling over, you can feel safer about a baby sleeping on the belly. It's very important to not have blankets that can suffocate in the crib. Try sleep sacks.