Don't worry, we don't care that you are younger than us! Welcome aboard.
I am glad in some ways that my spondy started when I was older. It was very mild when I was in my early 20's... I put it down to straining my back doing sports. Although I was never sure how it was that exercise actually improved it.
I would have hated to be your age with full blown spondy. I was a complete party animal at Uni. LOL
All I can tell you is how I go about socialising these days. It is a bit lacking in spontanaeity, but you don't have to diarise.... you are likely to know that Everyone goes out on Fridays, Thursday or whatever. Just try to make sure you have an easy day beforehand. Save up your energy. You probably know how much you can drink if you are taking your meds at the same time. These days, my tolerance is really low and I get headaches easily... so its only a couple of small glasses of wine for me. The equation SPONDY + HANGOVER = HELL anyway, so I'll bet you don't overdo it often. Just try and enjoy yourself the best you can -- and write back and tell us about it!
Anyhow, don't feel out of place here, you do belong!