*Siiiiiiiiigh* I swear, we have to have at least one of these every few months at the most...

The Kingdom Hearts universe is COMPLETELY different from the Final Fantasy universe. All of the people in Kingdom Hearts grew up under different circumstances. Obviously, Aerith wasn't killed at any point.

Besides, just look at the other characters. Tidus as a kid in the Destiny Islands, growing up with Wakka, Selphie, Sora, Riku, and Kairi. Not to mention that Selphie's a kid while Squall's a couple years older.