My wife and I are in our mid/late 30s and American and I'd say I we go to a proper party hosted by friends or host one ourselves at least once a month. In a "third place" we probably socialize another 1.5 times a week with friends at places like bars or meeting for dinner. This seems to be the norm around me. What's the difference? We live in NYC. It's very easy for us to get around, there's no drunk-driving and there are of course many opportunities for social activities outside the home. Back in the suburban state I come from I've noticed this is not the norm amongst my friends who still live there.
We're in our early forties with no local partners and yes, compared to all the years we spent in suburbs, living here in NYC makes a tremendous positive difference. We can't overstate how much it's done for our mental health to have found community hangouts we can reach on foot in all of several minutes.
Also living in a dense urban neighborhood (in another large US city) and I have a similar experience. There are many bars and restaurants in walking distance, but also my friends and I host each other often. We know each other well, so we don’t worry too much about trying to come up with impressive dinners. I have some go-to recipes that I can scale up easily. Dinners are often spontaneous: group texts light up towards the end of the workday and whoever’s available will show up.
I think the real key is that we are all within walking distance of each other. Friends who live a ten-minute drive away (still close!) are also invited but rarely show up. Also, no one has kids yet.