I'm a member of Midland Gliding Club up at the Long Mynd in Shropshire. I'm 21.
Yes, there are a few junior members ('juniors' in the gliding world means under 26) but due to the nature of the sport there are many more of the older generation who have retired or whose kids have left home etc.
You seem to think that this is a bad thing; but they'll have a lot to teach you! It's not like going out for the day with your parents: You are all there to fly, and most of the older lot are really keen to get younger guys involved - if you're not a complete twunt then the age difference is irrelevant; they just become your pilot mates from the gliding club.
However, the junior gliding scene in the UK is actually thriving. The UK Junior National Championships held each year (Lasham in 2014) are huge fun; ten days of moderate-to-heavy drinking, youthful antics and of course, excellent flying.
Throughout the winter, 'UK Junior Gliding' (actually just a load of juniors) hold a few weekends at different gliding clubs where we can all meet up, engage in some more moderate-to-heavy drinking, youthful antics, and excellent flying.
Check out this excellent video put together by the UK Junior Gliding team which sums up last season nicely:
