For randonneuring, I think the key is a comfortable and reliable bike; how old it is is kind of irrelevant. You'll certainly find yourself in the minority if you show up with a vintage bike, but if it can take you the distance in the allotted time, I don't see an issue.

This past summer, I did a 185 km ride (not a brevet) on a 25+ year-old Bianchi steel road bike. There were about 250 other riders, and most had very modern mid- to high-end aluminum or carbon bikes. My Celeste frame stuck out like a sore thumb! But the bike performed beautifully. The hard, narrow tires and a crappy saddle left me a little sore by the end, but with a couple changes, I wouldn't hesitate to use that bike on a 200 km brevet.