>>3339176Most of the fandom is guilty of not accurately portraying a 10yo -- /especially/ in dad!Reigen works. Realistically, that's a kid who's already developed legible handwriting in kanji, walks from and to school without adult supervision, probably knows a couple of curse words, and is reading chapter books. Also Reigen was the age of a fresh college grad student (23-ish) when he met Mob; adoption AUs are unrealistic in that regard, too.
I see the infantilized 14-15 year old Mob depictions more from the fujo side. I'd wager it to be fetishistic reasons, but most writers fail to decently portray how Mob emotes or speaks, so I'm not surprised.