I got married when I was 28. Started dating my wife when I was 25. Most of my friends that I am very close with got married around 25-26.
Personally, I don't think there is any "right" age to get married. Different people have different paths in life. I do think a ton of people get married for the wrong reasons at many different ages, whether it's 18 or 50. When the time is right, you will know.
But what you said is true. In high school and college, and even the 2 or 3 years after, I would go out most nights and not have a problem finding people to hang out with. In fact, I think I had too many people to hang out with. We'd go to clubs and bars in the southside and the strip district. And then when I got to my mid 20s, found my future wife, started dating her, my college friends were getting married, spent more time with them because we had a lot in common, and started losing touch with a lot of other friends.
Fast forward 10 years later, I'm married have one kid, and another on the way. I have a close group of friends all in a similar situation that we hang out with, rarely go to the bars mainly because of daddy duty and a busy work life, and about 75% of my friends from a decade ago I barely even talk to. Maybe run in from time to time, or see each other once a year, but you lose touch, go seperate ways.
It's just life.
As for if you want to meet people, I think you are on the right track joining different groups/clubs. I think you have a much better shot at finding someone there than at a bar. But then again, you might find that future someone at a bar!