Babies can safely sleep with a pillow when they are 18 months old.
Some babies have slept safely with pillows when they are at least 13 months old but doctors and pediatricians recommend for safety reasons that you wait until the baby is at least 18 months old before allowing them to sleep with pillows, blankets or anything else soft.
Babies should never sleep on any pillows including Boppy Pillows.
If a baby is 1 year old or under and is allowed to sleep on Boppy Pillows or any other pillows they may die of suffocation if they roll over onto the pillow and cannot roll back over.
Also the baby could die of positional asphyxia.
If the baby is just napping and you're supervising the baby then yes they can take a nap on a pillow or Boppy Pillow but never allow the baby to sleep on any pillows when unsupervised.
Also never allow a pillow to be in the crib with the baby until they are at least 13 months old or older and also never have any blankets or stuffed animals in the crib or bassinet with the baby until they are least 13 months or older.
You can use blanket sleepers to keep the baby warm at night.
Ideally you should wait until the baby is 18 months old before using pillows or blankets or soft toys in the crib with them.