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mahtan
About the way people treat him and his friends and the way they act not matching their age.. I think It's just a cultural thing. x:
Since they reach adulthood earlier, and are expected to be able to live and act as adults when they do, they are forced to become more mature earlier too. And on the MC and his friends' case this is even more evident. Bell for example, being the 8th son of a poor noble family, was forced to earn money by himself to pay for his own expenses since he was 12yo, or even before that. The same thing happened to Erwin, who is a 3rd son. And the girls, because their social status, they wouldn't be able to marry into influential noble families, so they also had to become independent at young age, in order to secure a good future for themselves.
Their society standards seems to be a similar to that of the early middle ages period of our world, which crontrasts with how advanced their society looks. The reasons for that are probably the remnants of that ancient civilization that had super advanced technology and the fact that they can use magic, so their technology and scientific knowledge ended up advancing much faster than ours, but their cultural values didn't.
My point is, on their world's society, at the age of 12 they're no longer "children", so they will not be treated nor will they act like it, because they're almost "adults". All things considered, since they already entered puberty, It's not strange at all the way they act, even from a biological standpoint.
And finally, about their appearance, thats just the character design that doesn't match their age, making they look older. Their world does't have a "level sytem" like on some other isekais.