Heya Charlie!
I'm also quite new to going to meets, I've only been twice on the Wednesday evening, but from what I can see the age range is fairly broad (20's, 30's 40's I guess) and the vibe is very relaxed and welcoming, so at 21 I'm sure you wouldn't really be out of place. I'm in my 30's but get on with a broad range people from teens up to old age, I try to see the personality and not the number, and that seems to be the mentality of others within the group meet-ups.
I totally relate to your description of avoiding social situations, that is also a large part of my anxiety disorder, so I'm trying to learn how to manage it by going to the meets, it was quite difficult for me to take the plunge and go to the first one, but I'm so glad I did and so far it seems to be doing be the world of good!
It would be great to see you there if you feel up to coming along, it's a great way to meet likeminded non-judgmental peeps!
x