Perhaps it would be helpful to quote the below notice (from a pre-flight services email I received this morning). My bolding draws attention to the salient points.

Air Passenger Duty (APD) charges for 12-15 year olds


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.
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What’s changed?

Since 1 March 2016, children aged 12-15 no longer pay Air Passenger Duty (APD) when travelling in economy. This encompasses all British Airways flights departing from a UK airport. Some affected customers may have been charged the adult APD and be eligible for a refund.

2-11 year olds stopped paying the tax on flights from 1 May 2015, while children under 2 years old have never been charged APD.
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Am I due a refund?

If you are travelling with 12-15 year olds (who are at least 12 and under 16 years old on the date of departure), you may have been charged the adult APD and be eligible for a refund in the following circumstances:
•flight-only bookings – booked before 30 September 2015 and travel from 1 March 2016 in economy
•flight packages (flights booked with a hotel, car rental, experience or transfer) – booked before 25 February 2016 and travel from 1 March 2016 in economy
•Executive Club reward flights (paid with Avios) – booked before 12 May 2016 and travel from 1 March 2016 in economy

In all other circumstances, including bookings with 2-11 year olds, you will have been charged the correct APD.




Travel agent bookings – if you booked through a travel agent you will need to contact them directly to find out if you paid the correct APD or are due a refund.




Transferring within the UK – if your ticket includes a transfer through the UK within 24 hours, no APD will have been charged so you are not due a refund.




How do I get my refund?

To qualify for a refund, you must provide us with the dates of birth of the 12-15 year olds at least 72 hours before departure.

You can do this in two ways:
• If you are travelling on a flight within the United Kingdom or to Jersey or the Isle of Man, or you're have difficulty in Manage My Booking, please contact your local service centre.
• In most cases, you can enter dates of birth under Advanced Passenger Information in Manage My Booking.

Your refund will then be processed automatically. The refund will be applied to your original payment card and should appear on your statement by the end of the following month.



* If you do not identify the young adults travelling by providing the information at least 72 hours before your flight, and the young adult status is not recognised, they could be deemed eligible to pay APD, and we will be unable to refund.