I raced as an expert cross-country for years. Lot's of fun. I still do it, but no longer have the desire to race.
Here's a shot at my local trails (Santa Cruz, CA area). I talk the whole way down, which is annoying to some, so SOUND OFF!
Alan's MTB ride - YouTube
Also, don't give up just because you think you're "old." I have had the pleasure of riding with Jim "Sully" Sullivan (first MTB World Champ).
GalbraithMt.com Snapshot Views: First World Mt. Bike World Championships, Durango, CO. He was REALLY fast even well after he turned 40. SUPER nice guy.
I also had a chance to race cyclocross against Thomas Frischnecht (second photo). I got some great shots of me in front of him (when he came around to lap me! LOL).
Point is, cycling is something you can do well in to old age. I plan on riding until I die. Generally speaking, MTBers are great people. Racing is also a load of fun, no matter where you place. You'll always find a group in your fitness/skill range to race with/against. Some of the most fun races I've had were when I "bonked" and just had a great time.
Al