From the BA site, for BA travel,
https://www.britishairways.com/trave...hy_young_adult
Originally Posted by BA
Passengers
Please specify the number of passengers based on their ages at the start of the journey. Please note:
•Young adults (12-15-year olds) – these bookings must include an adult when booking online. Young adults can travel alone but these flights can only be booked over the phone. Please call us to discuss your requirements.
•Children (under 12) – cannot travel alone and these bookings must include an adult.
•Infants (under 2) – these bookings must include one adult for every infant when booking online. This is because infants are seated on an adult’s lap instead of getting their own seat. To book a separate seat for an infant, please call us.
So for BA, book an 11 year old as a child , even though he or she will turn 12 during the travel
If booking on another airline, such as AA, then need to check with that airline for what its policy is
Originally Posted by mmxbreaks
Worth phoning BA. The APD would be refundable for an adult ticket, if purchased, but over the phone they should get it all correct.
Not needed for the OP in this case, but If booking for a 12-15 year old, the BA site caters for APD in regards to those 12-15 - select "Young adults" rather than "Adults"
There is no need to telephone