Whenever a site says that documentation is not required for infants, I do the exact opposite. Having documentation (birth certificate, doctor's letter, passport, etc.) will always come in handy. As much as you keep updated, you never know when the airline will change its policy. If the baby's age is questionable, expect to be asked about the birth certificate. There were some awkward moments during boarding, and let's say that it wasn't pleasant. To prevent these kinds of situations, check on IATA for further info before booking your flight.