One idea: Phone BA within 24 hours to cancel the booking for a full refund. You can only do this by phone. One reason for this cooling off period is to allow people to correct mistakes that they've made while booking - and your case would seem to fall within the intention.

I'm pretty sure that this applies to bookings made with Avios as well as to cash bookings.AIUI, 12 is the standard or conventional age to become an adult for most fares. But not every airline is obliged to adhere to this standard/convention.