That’s exactly my point though, it’s not juvenile. There’s nothing about that form of wit that’s specific to young people or kids. Indeed, it’s common in British comedy and if anything that particular example is the kind of joke common in 1960s comedy, and was usually between thirty something or older actors. That specific sort of repartee came back in the 1990s with stuff like Seinfeld and Buffy, but again that doesn’t make it juvenile.
I get that you’re grasping for an explanation for why you dislike this kind of back and forth stuff, which is defiantly un-serious, and popularized in genre stuff by Whedon, to the point where it’s sort of a cliche, but juvenile is just a mischaracterization, a miscategorization.
Whedon-esque is probably the correct categorization, and it’s fine to dislike that, I just think inaccurate shorthand like “juvenile” is quite unhelpful in discussing the flaws, real or perceived, of this kind of writing. A game with “juvenile” writing is typically one that is shallow and showy, with no subtext and perhaps a preoccupation with toilet humour or the like. That’s true of Divinity Original Sin 1 in all regards, for example. The writing is loud, crude, shallow, obsessed with poor quality humour and has no reflection or subtext.
Whereas as I said, in Anthem, Owen, who is the most Whedon-esque in his speech, has significant subtext, frequently using humour or wit to mask anger or fear. Even if you dislike the effect, some of the writing in Anthem is significantly better on a technical level than most games. Of course some is the usual RPG drivel - particularly with very minor stand around type NPCs.