I'm 33.
I had a lot of friends when I was in college. I think I hung out with like four different groups of people, and I'd often introduce people in different groups to each other.
After I graduated, everything slowly fell apart. As the years went by, we all slowly but surely drifted away. I'm at the point where I hang out with very few people in real life anymore. I have one very close friend from college who I still go out drinking with every few weeks, but he's it. Facebook has been a life saver here: I still talk with old friends all the time on Facebook, but our friendships are mostly confined to Facebook. Every once in a while, someone will say to me "We should totally hang out again sometime!", and I'd be like "Sure, that sounds awesome!", and nothing ever comes of it. I don't think there's anything malicious involved; I just think that both of us are too socially awkward to actually schedule a time and a place to meet up. There have been a couple of exceptions, where "We should totally hang out again sometime!" turns into concrete plans, but those are few and far between.
And it's worse if they're married and doubly so if they have kids. I think, before I go any farther, I should point out that I have no interest in ever having children, and I'm aromantic, which means that I am mentally incapable of feeling romantic attraction for another person. Though I'm technically a lesbian, I'm functionally asexual just because I have no interest in being part of a couple.
Take the guy I used to call my best friend, for example. As soon as he met the person who eventually became his wife, I never saw him alone again. This was a guy who was like a brother to me; we'd often stay up late at night and talk to each other about anything and everything, and we'd often confide things in each other that we'd never tell anyone else (for example, he was the first person I ever came out to). But as soon as he met her, I couldn't hang out with him without his girlfriend/fiancee/wife tagging along. Hell, when he got engaged, I invited him out to a bar so I could buy him a glass of scotch to congratulate him on his engagement, and he brought her along there! It got to the point where I couldn't have a serious conversation with him. It also really annoyed me, whenever we'd hang out at a bar, she'd decide she was tired and wanted to go back. They'd tab out ASAP and leave as soon as their credit card came back. I am, by the way, talking about a bar that's literally right across the street from their apartment. They'd disappear before I could even finish my last drink... before he met her, when one of us would get tired and decide to end the night, we'd at least both stick around until we finished our last drinks and then go home at the same time.
Or, for some of people I know who have kids, their kids are all they talk about. I've had to unfollow a lot of people on Facebook because their Facebook feeds just become constant streams of pictures of their kids and updates on their kids' lives (the only person I make an exception for on this is my cousin, and that's only because her kids are my family too). I've heard from a lot of people my age that they make most of their new friends by befriending the parents of their children's friends, so being childfree that completely cuts off the main avenue of making new friends for people my age (and honestly, it sounds like a terrible way of making new friends, where the only thing you have in common is that your kids like each other).
Making new friends at my age sounds more like pretending than anything else. I dread the idea of joining someone's existing friend group and struggling to understand all their little in-jokes. And even when I do understand them, they won't have any emotional impact on me because I wasn't there when the joke started. And if they're my age, they're going to be spending a lot of time reminiscing over the good old days, and that will always make me feel left out because I'm the newbie who didn't share the good old days with them. I'll always be the fifth wheel.
And then there's the fact that some of my former friends have become genuinely shitty people. Take my former best friend who I was talking about two paragraphs ago. Over the last few years, he's gone down an extreme alt-right rabbit hole. He'd always been a libertarian and a Christian, but he was always one of the good ones, and now he's gone full blown religious right extremist. His Facebook feed has largely consisted of memes with transphobic slurs, Nigel Farage videos, videos about how gay people can't get into heaven, screeds about how Jimmy Fallon's son deserved to die, the "piece of shit" meme about single mothers, etc. He went from being one of the most caring and compassionate people I knew to an utter monster. And, yes, his wife is partially responsible for this. She's an extreme tradcon who believes that a woman's highest calling is to be a 1950s housewife, and she's brought out the absolute worst in him. Eventually, after realizing he's too far gone to ever come back, I started ghosting both of them. I haven't spoken to him in over a year, and I actively avoid going to places where I know they like to hang out. I do, however, take a perverse pleasure in watching my count of mutual friends with him on Facebook drop like a rock. And they're all from people unfriending him. One of those people was a groomsman at his wedding, who publicly told him to "fuck off" after he posted something really heinous. The only good thing to come out of this though is that him going off the deep end has rekindled my friendships with his exes. The last two times "We should totally hang out again sometime!" actually resulted in something concrete happening have been with two of his exes, and we bonded over how he's turned into a horrible person. One of them even said that meeting me was the best thing that came out of dating him (and she's one of my friends who has a kid, but she's one of the few people I know who has kids where I can have a conversation with her about something other than her kid).
But, honestly, losing the man who was at one time the closest thing I had to a brother fucked me up really badly. I don't have anyone who I could talk to the way I talked to him back in the day. We'd been through so much together, and he'd been right there with me for every single important milestone in my life since I was 19. We got each other into music, TV shows, hobbies, etc. He got me into goth music, and I got him into Doctor Who. I took up photography for a time because I wanted to be like him. I can't replace that. There is nobody I can meet tomorrow who I will ever have that kind of deep friendship with, because all these important milestones in my life have already passed, and I'll never get to go back in time and experience those milestones with someone else. I have one close friend left (the guy I still go drinking with every few weeks) who I went through a lot of these milestones with, but I never spent nearly as much time with him as I did with he-who-shall-not-be-named, and our relationship wasn't quite as deep. Oh, don't get me wrong, it's deeper than most other friendships I've had, and he's far more than just another drinking buddy, but not to the extent of thinking of him as my brother. I'm just not going to find another brother (or sister) at my age, as it's the kind of relationship you have to start cultivating when you're very young. The way I see it, I have another 30-40 years left in me before my body gives out, and I'm going to spend all of them without having the kind of friend I had from 19 to 31. I don't really know how I'm going to deal with that.