Interesting. I'm just one person, but: I've enjoyed stable employment as a programmer since I was 23, and my teenage years and early 20s were easily my most socially (and romantically) active, and I rarely felt lonely then. I've felt most lonely in my late 20s and early 30s.
I sometimes wonder if my career destroyed my ability to make and keep friends.
Social groups just dry up when you get to your late 20's as people are marrying off, having kids, and only maintaining the utmost of friendships. I know it's brutal but it's a time thing... significant others and families take lots of time if you're doing it right so socializing time becomes of higher value/expense.
It was way easier to meet people when I was younger - not just romantic partners. Social groups were much more cohesive and much less based around couple activity. People were just down for whatever and would jump at something just for the experience. I had tons of friends I could just call/SMS like "hey I'm bored let's go find a show" whereas I would feel inappropriate doing this as an adult.
I think late 20s is when most people lose touch w/ remnants of their college social circle if they're not naturally outgoing. Usually everyone has moved away or married off at this point. It's fairly normal.
I sometimes wonder if my career destroyed my ability to make and keep friends.