I get this question often in my office. There is really only one thing you need to be aware of while taking an infant on a flight. Babies can’t “pop” their ears to relieve the air pressure like adults can. This can cause significant ear pain and can turn a happy baby into a very fussy one for the entire flight. You can help balance the air pressure in baby’s ears by breastfeeding during the entire ascent, and also during the entire descent. The descent is more important as the changes in ear pressure can be more painful at this time. Alternatively, you can let your baby suck on a bottle or a pacifier, however, the increased jaw motion during breastfeeding is better to keep the ear pressure equalized. Bring a bottle of Tylenol to give baby if crying gets excessive.
Medically, babies can fly at any age, even at day one. Airlines may have their own policies on how old a baby needs to be to fly.
Many people recommend giving benadryl to infants during a flight to ease the pressure in the ears. Benadryl does not equalize ear pressure. Its main effect is sedation. So while it seems to help with the ears, in reality it is just putting baby to sleep.
You should have older children chew gum during the ascent and descent to keep the ears clear.
It is also important to keep in mind that the safest place for an infant during take off and landing is in a car seat securely strapped in. While this does require you to purchase a ticket for the baby, it is simply safer. Once the plane is in the air, you can take baby out to nurse. On desent, secure baby in the seat a few minutes before the plane touches down.