1 year. The american academy of pediatrics recommends keeping all blankets out of cribs until at least 1 year old. This is in order to decrease the risk of sids (which dramatically decreases after 12 mo). Instead of using blankets for your young ones dress them in warm footie pajamas. After 12 mo most babies have the ability to remove a blanket from their face or roll over.
Sleep. Should be fine as long as closely monitored and does not gag or suffocate on self.