You can place your baby on their stomach to do what's known as Tummy time as soon as they're 6 to 8 months of age but that should only be done while they're awake and when you're supervising the baby.
However you should not let a baby sleep on it's stomach until it's at least 13 to 14 months old which is when the baby can usually turn it's self over.
If the baby is placed on it's stomach to sleep before 12 months of age then the baby has a higher risk of SIDS so never let your baby sleep on it's stomach until at least 13 months old.