Those meetups where everyone is decades older than you? Many of them have children that might be your age. Make friends with both. Some of my best adult relationships are from older folks with similar interests introducing me to their children.

I do agree about the general uselessness of Meetup.com though.

My first suggestions is and will always be: "try joining a martial art dojo/group/team/association". Martial arts range the whole spectrum from strenuous to fairly gentle so it does not matter if you are (or want to become) very fit/competitive or if you just want to relax and improve your general wellbeing and coordination. You can also pick zero-contact stuff like Iaido or Tai-Chi.

Assuming you like the activity (please allow yourself at least three months of constant practice before drawing conclusions) you will almost automatically pick up social contacts through that.

So you are in Germany then?

Just join any Sportverein! If you don't like exercise, then Chess or something. There are thousands of Sportvereine (and other types of Vereine) in each city, pick one (or three), go regularly.

I've had similar experiences in such settings. You might even find that you have some particular shared interests, might make plans to get together at some point, but nothing ever comes of it. I guess that is just human nature. My very particular piece of advice: If you are only and want to connect with people, don't limit yourself to Chaos events (or something similar). These gatherings are not very well-suited to making the type of connections you seem to seek. Even people who have been part of the scene for a long time struggle to meet people there. It's pretty much a scene like any other: If you don't meet the right people early on and make some kind of connection it's hard to get further and you will always feel left out to some degree.