I was diagnosed in my mid-20's and was also pretty active. In my case my spine fused top to bottom so I lost mobility and ability to do a lot of things. Good news is that 35 years later I'm getting ready to retire, have two sons, play golf decently (18 hdcp) and generally things have gone pretty well. You learn to accept your limitations and focus on the things you can still do.

I'm going to need a new knee due to arthritis but most of my friends without AS have one or will be getting one.

Pain for me was worst the first 10 years, then my body adapted and the pain subsided.