I'm not sure if this has been covered elsewhere, but I'd like to highlight BA's conditions of carriage which deny children their APD exemption when the booking is made within 3 days of departure. See this quote from there website. This is outrageous.

Air Passenger Duty (APD), a UK government tax on flights departing the UK, changed recently for 12-15 year olds travelling in economy. Depending on your type of flights and when you purchased them, you may be eligible for a refund. If you are eligible, you must take action at least 72 hours before you travel to qualify.