Other mistake: After completing a few tasks at Lon Lon Ranch, Link is rewarded with a dairy cow of his own. But somehow, the cow magically just appears back in his house back at Kokiri Village. His house is basically a tree house way in the air accessible just by a ladder. And the doorway is only big enough really for a small child to enter. Yet somehow a full sized cow is now in the middle of his house. And survives without any grass or water inside.
Other mistake: The guy you give the Rabbit Hood mask to will later challenge you to a race, him on foot and you on your horse. Once he leaves the starting area after the race starts, you never see him again during the race until you get to the bridge. Win or lose, you only see him at the bridge. If you won the race, you should have seen him as you passed him.
Answer: Because the Kokiri don't age past a certain age, but they do age up to that age. Link looked roughly the same age as the other Kokiri when his adventure started, which is why they never noticed that he was different.