Thank you all, and yes, ExXB, that was the point I was trying to make.
So if the seat reservation system has no idea of passenger ages, does that mean I can select seats that are all over the plane? What fun.
It does go some of the way to explaining who so many people were angry that they were booted out of the seats they had booked. I know the terms and conditions say you don't always get the seat you booked, so you have little reason to complain. However it does make the idea of booking seats a little ludicrous if the plane has be reshuffled because the reservation system cannot put minors with one parent.