I know a guy, 80+, who still rides (not as much these days), and he has probably been one of the most encouraging, welcome-to-the-sport, come-ride-with-us people I've met.

As for acknowledging other cyclists, I think it kinda depends on the situation. If I'm out for a leisurely ride and see someone else out doing same, we generally trade a nod. During a busy commute, it's all about communicating with other road users to stay safe. I don't expect much communication from people who are clearly on a training ride, other than for them to call out if they are passing me. I'd rather they focus on what they need to do to have a safe ride.