The problem is one of feedback. One who is extremely bad at conversation would likely be too socially unaware to recognize s/he is.
* Reading from a variety of sources (HN does provide a lot of this, click on more than just the tech articles).
* People in our age group seem to have more interest in being heard than interest in listening. Don't do that. Ask questions (which is in itself a good way to hold a conversation), even about things you wouldn't think you would be interested in.
* Learn to at least listen to opinions that differ from your own. Big bonus if you can dedicate energy to contemplating those opinions.
In addition:
* Nobody wants to hear about Game of Thrones or the latest computer game that you play. Entertainment media rarely makes for good conversation unless you share the specific entertainment piece in question (mostly sport); however, you are doing little for your future conversation ability by participating in an echo chamber.
* If you screw up people will probably forget about you, not remember you as an idiot. Don't be scared to practice on strangers.
* Keep your bloody phone in your pocket and ignore it.
This is really the only trick. People love to talk about themselves.
As the saying goes: "Interested people are interesting."
When people complain about uninteresting conversations, they often mean, "I wish that other dude would shut up so I could talk about the stuff I like!" They're not listening. If they were listening and interacting with what the other person has to say, they wouldn't be bored and frustrated.
Some people won't shut up at all, especially older ones. If you want to hear interesting things you might have to steer the conversation a little, so eg you can hear about how the city used to be in the past as opposed to how their doctor's visit this morning went. However, the most important part is to find the ones that want to talk and broadcast the same intent. (There is better luck when it's not just people going to/from work, those usually want to be left alone.)
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.)
Actually, I think you're both right. The idea that young adults should be suspicious and selfish is limiting and wrong. But the idea that older adults have different motivations socially is spot-on. In part, even the introverts and misanthropes have learned to value just talking to people, eventually.