That crossed my mind as well, and I did think of replying to the existing thread on that subject, but after a moment of reflection it seemed clear that it probably isn't related. Previously, everyone of 12 years of age and more was classed as an "adult" with BA. That is no longer the case.
It's just that I think I've heard that BA charge more for bookings made over the telephone, and I suspect they are using the removal of APD for 12-15 year olds as an "opportunity" to force people to book over the phone. There is no explanation as to why you have to do this; the web site just tells you that you must do it this way.