Infants at least 16 days old may be accepted for travel. Infants (less than 2 years old) travelling without seats must be held by an adult during takeoff, landing, turbulence, and when the “Fasten Seat Belt” sign is on. In general, doctors recommend you wait to fly until your baby's immune system is better developed. This could be as soon as one month for full-term infants, though most doctors recommend anywhere between three months and six months.
