It’s amazing how short people’s memories are. Not very long ago, it was not possible to book a Young Adult at all, it was either 12+ as an adult, or U12 as a child. Then the government came along and offered an exemption from APD for teenagers, and almost overnight airlines had to come up with a fudge to allow a new category. But literally nothing else changed… it’s still an adult fare, it’s still the same underlying ticket, it’s still the same systems handling the itinerary, it just that the ticket doesn’t have APD attached to it. And it’s totally optional.

In any case, your point would only apply up to and until the point of check-in. As soon as the passport is swiped, they’re fully aware of the passenger’s DOB.