Some popular subs can have good content, too - askreddit has a good topic once or twice a month, and depthhub (used to) have good curation of deep learning.
I disagree. Those subs are chock full of people who suffer from "expert beginner" symptom. The real experts have moved on to communities with a higher bar of entry (mostly forums). You can't have real in-depth discussion about technical subjects when you're constantly getting buried by people who are mostly trying to score internet virtue signaling points by pointing out that their table saw is old and lacks the most modern safety features or that they're not using jack stands. Forums (or 4-Chan, or anywhere else with linear, non-ranked discussion) doesn't have that problem. Once you're above the lowest common denominator in any sub there's little left to gain because you can't reliably interact with people who are at or above your level (because those people get drowned out so they start keeping their mouths shut or leave entirely).
I think it's just that, ultimately, younger people have more time for online community than middle-aged folks, and the average tone of most subreddits clearly shows this.
Perhaps I'd have a different perspective if I was pursuing a new hobby, so I was the n00b, but I'm.. not.
This was inevitable though. At some point the corporate overlords will want to cash out. Time for something new.
If you only browse /r/all then yeah it's very likely you will be in a room full of teenagers.
Go to /r/askhistorians to be in a room with academics that take themselves very seriously.
You can find anything you want there if you look long enough.
/r/starcraft: doesn't have nearly as much good discussion as TeamLiquid, but TL's forums are not easy to trawl through. Great coverage of tournaments/announcements tho
/r/personalfinance: really great here, lot of professional financial experts drop knowledge bombs. I've improved my financial life oodles just reading through this.
/r/piano: lots of great/interesting posts, mostly people playing pieces or shots of their set-ups, not too much memery but people try a little bit. Quite a few better/more experienced players than I am to answer my Q's.
The main thing I hate is memes. They are very low effort, many of them are inside jokes/highly meta, and the humor is usually bargain-basement, not very funny. If you can avoid those subreddits where memery takes over, then you have a better experience IMO.