Go for it
While I can't say that I have actually competed on an older steel bike, I am a construction superintendent that has spent the biggest part of 10 years living in motel rooms. The bike I travel with is an ancient Univega Viva Sport, and I always try to find group rides wherever I am.
Invariably, I ask a LBS where the rides are, and I show up with my ratty old travel bike. I am 47, and it is absolutely amazing how many rides I have ridden alone, because most of those riders out there with their $3000 bikes are not into it mentally or physically, and drop on the first hill or headwind. (Houston has some hellacious headwinds, and I'm not even going to mention Wyoming)
It is sort of fun to have everyone looking down their nose at you at the start, when you know you are going to outride 70 percent of them regardless of age or equipment.