when i was on a BA flight a few years back (i was 16) going to manchester on a Dash 8, i asked for a window seat and the nice lady said the only row left was the emergency isle and that i had to be 18 to sit at that window!
It did spoil my flight down as i had to sit with an mid aged man who had the window shutter down the whole flight!
BA surely has a policy for it.