No, that was what you meant. That's what you said. That's
all you said:
Make him a 18-year-old kid who looks like a child.
Well, that, and alternatively:
Make this her 18 year old brother who looks 10.
If it was about a supposedly childish characterization, you would have never left it at this, because that would still be a problem if nothing changed except his age (especially if you think he's no different than a 10-12 year old boy). But why are you even expecting me to hammer out your propositions to be as good as possible?
He already is technically 16 but lets be real, he's 10 or 12 in the way he acts, the way he's drawn, he's a child not a teen.
He's characterized as a high school kid trying to look cool in front of a working adult that doesn't even consider herself more mature than him.
This whole series is built off the "Loli who isn't a real loli and acts like an angry boss" logic.
...we are not reading the same manga.
Shoot, am
I lost?
Putting that aside, that still doesn't get to the crux of the issue, which is that you're agonizing over him not being exactly one or two years older, on
chapter 216 of this manga.
I don't even get the issue-- at most, he's impressing her, but they haven't even
held hands, let alone kissed.
Natsumi prioritized her own feelings instead of focusing on appreciating Futaba's happiness but Yuuto prioritized Natsumi's feelings instead of focusing on himself.
It's beautiful that Natsumi noticed this and rewarded Yuuto for caring about her feelings.
Damn, I didn't consider this too well. The "maturity" she's thinking about is probably in the context of what you mentioned, and is less an indication of the "general maturity" that'd be most relevant in terms of contributing to legitimizing an intimate relationship in the narrative.
I got carried away, only viewing one direction...