It's to do with setting a standard... ATC is a professional job akin to flying medicine, law, etc., and it's a bit more than a bloke sitting in a control tower. NATS sets standards, much as some professions require university degrees. If you are successful you will find yourself studying subjects during your training which you may never use... that's life, as they say.
Starting from scratch at your age with very little background may be difficult which is why they'll want to be reasonably sure that you can assimilate the knowledge required.
One other point. Having spent the last few years "swinging in trees", are you sure you want to be shut away in a control room for hours on end? Remember that with NATS you might get posted to an airfield but you might also be posted to Swanwick, where there is not an aeroplane to be seen.
Good luck