According to United Airlines, they offer to children fly as an infant if they are under two years old. Moreover, Infants can travel on domestic and international flights. However, domestic flights are free, and international flights charge 10% of the adult fare. Additionally, United Airlines Infant Policy requires documents such as birth certificates for domestic flights, but international flights should have birth certificates and passports. For more details, visit the Airlinesunaccompaniedminor mentioned blog link.