On the flipside, that is what makes me feel old.

In my teens/20s, my friends/colleagues and I would just go casually someplace to hang out after work/school without planning squat. (After class) Hey, let's go to x's house and chill. (After work) Everyone, let's go to x bar and unwind for a few hours. Now? I can't remember the last time I could casually say to a friend to hangout tomorrow. Practically everyone (save one's SO) seems like they need a few days to weeks notice in advanced. It's not an exaggeration to say that sometimes I have to book people 2-4 weeks in advance just to have a coffee/lunch! All I have to say is thank god for meetup.com which fills the space in between.

I had the same experience when I hit my 30s. I think it's because peoples' social networks expand through their 20s, and there's a bit of a social arms-race once peoples' calendars and social circles start to fill up. It's often considered rude to say "Somebody invited me to something else and I'd rather hang out with them than you, so I have to cancel", so most people take a first-come-first-serve approach to accepting invites. That means that each one of their friends needs to schedule before anyone else gets to them, so peoples' calendars fill up increasingly far out.

And then they get promoted, get married, and have kids, and they don't have time to hang out at all...

It's as much a cultural thing as it is a generational thing. In Argentina, where I grew up, showing up unannounced to a friend's house is normal (some people might add: "as long as you bring some food!"); on the other hand, in neighboring Brazil it was seen as a pretty rude.

So, I guess, what might not work for you might work for other people. I personally prefer at least a couple hours of heads up, but don't mind doing a last-minute dinner/lunch when I'm close to a friend I haven't seen in a while.