This feels like less of a case of 'American loneliness' and more a case of the author idolizing being in her 20s and projecting the same standards onto her 30s, when people typically have many more responsibilities.
Of course people who you don't even work with (and thus don't share a schedule with) likely won't be available on a whim. Even more so if they have young kids to plan around. Once the kids are a bit older, it gets easier again, as the kids can take care of themselves to an extent, and you can find common ground with and befriend the parents of their friends.
I often wonder about this. I'm older (50s!), and fondly remember my younger years, with a vibrant social circle. However, things are different now, the vibrant social circle is gone. But is this because we were all young and energetic back then? Or, was it a different social era (pre-Internet)?
... Or, am I just reminiscing about my "glory days", inventing a robust social circle, ignoring the fact that I was the same person then that I am now, with basically the same social life?
College seems to me to be geographically structured to help you make friends. I remember reading one study that shows basically repeated random interactions with the same person will tend to develop towards making friends with them organically. Well how does college work? You take classes in your major, where you will meet other people with a common interest. Also you might live in the dorms, eat at the mess hall, etc where you're also going to bump into the same people again and again.
I just turned 40 and have a very, very vibrant social life and am out doing things just about daily after work. But I also don't have children, live alone, and I live in a big city surrounded by things to do. My friends with children at this point have them old enough to be hands off (not their taxi/babysitter) or they're raising newborns.
I absolutely don't go hard like my 20s and I'm usually home before 10pm but that's more so because my job is in the morning and I need my brain power to do it good well enough to keep it.
I have a lot of friends in their 25-50s that live in the same neighborhood as me or are within a 5-10 minute bike/walk/uber.
So there's definitely people outside of the stereotypical age range still socializing heavily. I see the same bicyclers riding together all the time here and they look in their 40-60s. I go roller skating and play hockey with some beer leagues and almost everyone is around 40.
With all of my free time and being trapped at home with cabin fever I've taken up a lot of my old kid hobbies like hockey and skating, playing guitar, etc.
I imagine it's a mix of all that. I'm only in my late 20s. I remember in my early teens, when internet access was still mostly something done at a desk and giving children a cellphone with service was not as common, I had a large vibrant friend group whom I used to spend a lot of time with. Though in hindsight I don't really remember it fondly since the friendships were kind of toxic and not as deep as with my 2-3 current friends.
During college I didn't really make friends, mostly just saw them as acquaintances since we mostly didn't really have much in common.
I only really interact with my current friend group over the internet and we've been sharing almost everything with each other for pretty much every waking moment for the past 5-6 years. I consider this to be fulfilling my social needs pretty well, particularly since I'm not all that outgoing in the first place. What I am missing out on are ancillary benefits of physically present friends, like being able to ask them for favors.
Of course people who you don't even work with (and thus don't share a schedule with) likely won't be available on a whim. Even more so if they have young kids to plan around. Once the kids are a bit older, it gets easier again, as the kids can take care of themselves to an extent, and you can find common ground with and befriend the parents of their friends.