Lap infantsUnited Airlines, Delta Air Lines, Southwest Airlines and many others have similar policies for domestic flights, with a cutoff age of 2 years old. On international flights, there is no fare for lap infants, but fees may apply. In most cases, you'll need to add the infant to the adult's ticket.
Does a 2 year old need a plane ticket on Southwest?
One child over 14 days old and under two (2) years of age not occupying a seat may be carried free of charge when traveling with someone 12 years of age or older. If you plan to travel with an infant as a Lap Child, you must pay the taxes and fees that apply to the international portion of the infant's itinerary.
Can a 2 year old sit in a car seat on an airplane?
Bringing a car seat on the plane isn't required, but it's the safest way to fly with your young child. Though children under 2 are allowed to sit on your lap for free on domestic flights, you may decide you'd rather buy your baby or toddler a ticket so they can fly in their car seat.