Itachi and Kakashi are both said to be geniuses, and they graduated from the ninja academy at really low ages. But geniuses like Sasuke and Neji were not able to do so. Why is this? Is the difference between the talent of the first pair and the latter really so great?
The reason for this is actually because of a change in the policies regarding graduation at the academy. Kakashi and Itachi were able to graduate early because of the ninja war, which necessitated a great need for ninjas such that any useful young ninja needed to be put to use right away.
But the war was over by the time Itachi became a ninja, so how can this be? Actually, there is a reason. There was a time lag before the policy was changed, and that's how Itachi was able to graduate early.
My source is the Itachi's Story light novels. From the second book, page 43, chapter 4:
"There's clearly a reason why they called you a prodigy when you came to the academy," Kakashi said.
"I'm not sure if the youngest graduate ever can really say anything here."
"When I graduated, it was in the middle of the Great War, and they needed ninjas. The situation's different now."
Now that Kakashi mentioned it, the current system at the academy was different from how it had been when Itachi graduated. The Great War and its aftereffects still lingered when he graduated. Because of that, once his actual abilities were recognized, he was skipped ahead, leading to his early graduation. But now that it was a time of peace, the Hokage was determined that ninjas must be carefully cultivated over a number of years, and it was no longer possible to graduate in a short time, as it had been in the past. Thus, no matter how talented Sasuke was, he couldn't become a ninja until he was eleven years old.