It depends on the destination you are traveling to.
Adults are required to produce their original identity cards for domestic and passports for all international flights.
Children may be required to produce their birth certificates or photocopy before they are allowed to board.
The general rule for international travel is that you must ensure your passport is still valid for at least another six (6) months at the end of your travel date, the applicable visa(s), and a return or an onward journey ticket. The system can't block you from purchasing the ticket as it won't be able to track the requirements you need to travel.
For further information & other destinations, please visit IATA’s Travel Center site for Passport, Visa & Health travel document requirements: www.iatatravelcentre.com/passport-visa-health-travel-document-requirements.htm. Here is an alternative site where you can also check for entry requirements, please click here: www.united.com/web/en-US/apps/travel/passport/default.aspx?SID=C4EA7800557D4DB6B61B353EE26151A5.
If you can’t find the information you are looking for please contact your local immigration and custom authority or the embassy / consulate of the destination you are traveling to for further assistance.
Remember to observe the immigration requirements for the destination you are traveling to as we reserve the right to refuse to carry you should you fail to produce the necessary documentation during check-in - we are quite serious about this so please check that you have everything in place before you travel.