Chris P has a point. If by "young" we mean "young enough that you're not likely to own a car", a festival as inaccessible by public transport as Rothbury is and with no walkable accommodation is never going to attract them. (By that criterion I'm nowhere near old enough at 59). Would Glastonbury ever have taken off if there's been a seven-mile walk from the stage to the campsite?

I've already posted this link in another thread, but here is my report on two festivals I went to this summer which, while they were quite small, had no problem at all attracting young people (look at the photos):

Hungarian music and dance camps in Romania

There are lots of those all over Hungary and Romania all summer.