I have been doing a little research into local naturist clubs in the Yorkshire area. There doesnt seem to be much to offer for younger people unfortunalty. I had a look at this club and again there isnt much that I would enjoy other than playing darts and pool and chilling in the bar. There is a outdoor swimming pool but around here its unlikely I would get to use it. Here is a link to some video clips and the website if you want to check it out. It looks like a nice, peaceful place to relax. But that just isnt my thing.
http://www.whiteroseclub.com/whiterose/video.htm
That is, I'm sorry to say, true of nearly all the clubs I have been to in the UK. Even ones I rate highly (like Spielplatz) really only offer a bar/clubhouse, swimming, pool tables, and sports courts other than open space to lie around in.
These aren't really age-related activities but they can be boring if you are the only young person.
I would recommend joining YBN through getting a BN membership - you might be visiting the same clubs but you will be doing so with other young people and that can give you a better chance of making your own entertainment and just chilling out and socialising with others your age while nude.
If you are intending to visit any clubs independently next summer and you want some company, drop me a message, I'm always up for a game of naked pool!