The anime is Genshiken (2004), I picked it up on a whim. It's alright so far. Very "by weebs, for weebs". The manga is from 2002.
The 80s and 90s being a pass of the baton from veterans of the early days to more modern staff, which already grew up with anime, and the success of Evangelion and merchandising had some awful long-term consequences even if it was probably unavoidable under capitalism.
I've said before that a lot of it does feel like Hollywood casting late-20s/early-30s actors for teenage characters, because you get characters that scan as like 20 something and act like adults, but then the narrative makes them like 14 for no reason. You also get characters that scan as 30-40 being assigned ages like 20, too. It's weird, like artists are compressing a multi-decade-wide range of adult character designs down into a range of six years and labeling all of them as teens for no story-related reason.
Someone else pointed out that at least some of it is adult writers basically writing a story with adult characters and then aging them all down to make them more relatable to an intended teen audience, but without actually changing their appearances or behavior.