Considering that Eorzea is 'exactly' the size it is despicted in the game shuts down a lot of potential roleplay. We can assume Eorzea is actually much, much bigger, but the game has to 'scale it down' for gameplay purposes: nobody would like spending three real life days travelling from one city to the other by foot. So there's plenty of 'free' space for small meaningless towns and random unruled wilderness.
Now, if by 'growing up' you mean literally, growing up from a little boy to an adult, I'd say that person wouldn't have any kind of spoken language at all and, what's more, he would a lot of problems fitting back into society. He would be very animalistic. I guess you could compare him to Tarzan.
However, if you meant that he used to have human contact before but that at some point he was left to 'live secluded for a long, long time', then it's plausible if he was in a very, very obscure location or if he chose to avoid human contact.
In that case, him having no concept of 'yours' or 'mine' can make sense or it can make no sense depending on the place. If there were predators or other people (whom he avoided) that tried to steal his food and were very territorial, then he'd have a VERY distinct and primitive concept of 'mine': he'd be very violent when other people put their hands on his things. Ironically, he would probably see stealing as another means of survival, so even if he has a very strict concept of 'mine', he would have, perhaps, little respect for what is 'yours'. Or maybe he would respect it a lot, identifying himself with other human beings and not daring touch anything that 'belongs' to others.
Now, if by 'growing up' you mean literally, growing up from a little boy to an adult, I'd say that person wouldn't have any kind of spoken language at all and, what's more, he would a lot of problems fitting back into society. He would be very animalistic. I guess you could compare him to Tarzan.
However, if you meant that he used to have human contact before but that at some point he was left to 'live secluded for a long, long time', then it's plausible if he was in a very, very obscure location or if he chose to avoid human contact.
In that case, him having no concept of 'yours' or 'mine' can make sense or it can make no sense depending on the place. If there were predators or other people (whom he avoided) that tried to steal his food and were very territorial, then he'd have a VERY distinct and primitive concept of 'mine': he'd be very violent when other people put their hands on his things. Ironically, he would probably see stealing as another means of survival, so even if he has a very strict concept of 'mine', he would have, perhaps, little respect for what is 'yours'. Or maybe he would respect it a lot, identifying himself with other human beings and not daring touch anything that 'belongs' to others.