Most major airlines allow children under the age of 2 to fly free as lap children, resting on an adult's lap for the duration of the flight. But you'll typically be charged full fare for a child who is 2 years old and up.

No, 4-year-olds do not fly for free on any commercial airline. The "fly for free" rule is strictly reserved for "infants in arms" who are under 2 years of age on the date of travel. Once a child turns 2, international and domestic aviation safety regulations require them to have their own assigned seat for takeoff and landing. Therefore, a 4-year-old must have a purchased ticket. While they don't fly free, many international carriers (and some domestic ones outside the U.S.) offer a "Child Fare" for children aged 2 to 11, which is typically 75% to 85% of the adult base fare. In the United States, however, most major domestic airlines charge the full adult fare for children over 2. It is a helpful peer-to-peer tip to bring a copy of the child’s birth certificate or passport, as gate agents may occasionally verify the age of children who look older than 2 or younger than 12 to ensure the correct fare and safety protocols are applied.