Have you played Crisis Core? Also Nomura probably didnt even think of Zack back then and does it even matter?
Nomura didn't even think of Zack?
Okay, no, no, no.
Nomura thought of Zack long before Kingdom Hearts came out, namely in 1994, when development for FF7 started.
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As for the actual post, I think you guys are counting on the fact that this Zack and FF7's Zack are the same person.
Tidus, Wakka, and Selphie were all younger in KH1.
Yuna Rikku and Paine were fairies, for God's sake.
I think it's safe to assume that this is just Zack's current age in KH, because this is KH's Zack, just like how KH's Tidus never knew Yuna and is fourteen.
It's probably even safer to assume that he won't interact with Cloud's character at all. Personally I think it's way more likely that his entire relationship with Cloud will be redone with Hercules playing Cloud's part, seeing as 1) it's in OC, 2) Hercules is young, 3) Hercules "becomes a hero" after Zack leaves OC or dies or whatever.