Yes, Young People do indeed take photographs with their mobile phone. And in galleries such as the Wallace, the Louvre and so on, young & older people take photographs with their professional cameras they mostly do not know how to use. 

Although I understand your frustration in being prevented of taking a picture of England's new Titian, I have seen too many times people taking photos of a work and then looking at the work ON their camera, instead of looking at the work itself. Or people taking over a work because they want a picture of themselves NEXT to it (apparently taking a photo is not a proof of presence anymore, they have to be on the photo to make it clear). In very popular galleries, this tends to be particularly annoying, very much like in concerts when half of the audience is more preoccupied filming the show than actually living it. 

Therefore, as a fairly Young Person and soon-to-be-curator, I hope the prohibiting measure will continue at the National Gallery and in other museums. Even though it means I will sometimes be frustrated when seeing a work I'd like to keep visual record of.