@Tersione
Well, regardless of what his age would translate to in human years, he's still plenty inexperienced about the world and wholly unsure of himself. His demeanor also makes him come off more like an awkward teenager than anything else too.
Arguably that could describe people in their twenties too. It's been some time since I've had a "teen" in my age but I still haven't graduated from being a young adult. (That, or I'm getting defensive cause he reminds me of me.) Also I imagine the best way to become uninexperienced about the world is to experience the world. Also not growing up in Japan, I have this romanticized notion that anyone of sufficient age is free to roam as they please, with the freedom afforded them by an
actual public transportation system.
I'm not sure how much we'd get out of a discussion with his "immediate" family; they all seem laissez-faire raising him, so I imagine it'd be a pat on the back, a "that's great" and a "see you later". It's actually a bit strange since they're probably young parents (judging from Takumi's age) and I imagine would be at that stage where they're overprotective of anyone they considered their child... but I've never had a kid, so what do I know? Maybe there'd be a discussion with his youkai family, but given that Jirou and Gramps/Tazenbou urged Buchio to visit Sanmoto-san the first time they met Buchio, I'm guessing that would've been a foregone conclusion.
I did feel it weird for there to be a one-off for him going to Tokyo. Maybe noho-sensei could've expounded on his "first time"s with him trying to figure out how to navigate a totally different environment. But I also feel like noho-sensei decidedly omitted the more mundane parts of the human experience, though now I wonder what a youkai train station looks like. You could argue that the whole romance thing with the kappa girl is pretty mundane too, but I like romance.
Anyway, sorry if the comment is overly long. I'm trying to sort out my feelings about this manga too.
P.S. in typing this comment, I remembered that 山木-san is Sanmoto-san and not Yamagi-san, so I've hopefully fixed all the occurrences in the chapter.