Not sure of your statement "Non-police/military/fireman" has to do with your questions. At any rate, I retired (from the military *and* w*rking all together) a few years ago at 40.
I am like some of the other posters that I don't have a ton of friends, so that isn't really too big a deal. As a matter of fact, since there are only a handful of people that I really enjoy hanging out with, it has been the perfect scenario for me. I am available just about all the time to do whatever with friends, but my social calendar is pretty open...which I prefer.
As far as the age group, I spent a lot of time with my Dad the last few years (he recently passed) and as a matter of course, he had a "Sunday Happy Hour" without fail and I was able to enjoy the social aspects of that with him and several of his "traditionally retired" folks that were well into their 70's and 80's. Sadly, most have died or moved on, so I don't have that social interaction anymore, and it was really fantastic.
As far as what "I do", well the easy answer is "whatever the hell I want". It sounds very simplistic, but it's the best answer. Some days I don't have anything planned, and others I do. As an example...I was running up to the store a couple of days ago and heard the Braves baseball game on the radio. The commentators mentioned that since it was a day game and school was in, the ball park was nearly half empty. Well, it was the bottom of the 1st inning and I thought "why not?"...so I went to the park, bought a ticket and enjoyed the rest of the game and a few cold beverages. It was fantastic and having the freedom to do something like that is...well, you can't really put it into words.
I have often heard that you should retire to "something". I am still not sure what that really means and I didn't do that. All I know is that I wanted the freedom to do as I wish and retirement was the way to do that.
Last comment...on Mr. Money Mustache. I will agree that he has some good ideas (but can be quite brash), but I don't think I would call him retired at this point (I know, I know...internet police!!!) as he has a LOT going on right now. He does mention that he doesn't *have* to go into MMM HQ on a daily basis, but I think he's lying to himself. I think he is full on "FI", but I question the "RE" part of it all. Then again, I grow very wary of the "FIRE movement". My Google feed is replete with stories of those who "FIREd" that is rehashed over and over and over and over again. And the terms, "lean fire" "barista fire" and "side hustle" all make me want to vomit. I should post the email I sent to the NYT when they asked me if I would answer some questions for yet another upcoming FIRE article...I made sure that no one will read my story 37 times over.