People in their 20's are engaged primarily in mate selection an career building. Those are very focused and somewhat desperate endeavours, where random conversations with strangers are high risk and low payoff, particularly when talking with people in your own age group who are likely to want something from you, just as you are likely to want something from them. It's like being at one of those networking events where everyone is there to make connections that can help them but aren't able to offer much in return: an unpleasant waste of time.
Older people are much more pro-social. Their investment in their offspring will be enhanced by a more friendly, connected world, and they can afford to indulge themselves in random social connections. Human society is fundamentally kept intact by grandparents. The reason we have these ridiculously long lives is most likely because troops of early humans who had more grandparents around were better at transmitting culture to new generations.
I'm putting this in brutally evolutionary terms, and there is obviously much more to the story, but the evolutionary forces are always there, under the surface, and we can't just wish them away (however much we might like to.)