Sasuke and Sakura are good and smart - but that doesn't mean they were as good before the year they took the academy exam, which would mean soemthing like they didn't feel they were ready to take any exam before then.

In Naruto's case, he believes he's always good enough, which would mean he'd probably attempt the exam every chance he got. It seems you can take the exam any time you want seeing as there are several cases of exceptional ninjas of a young age. His classmates would think he's an idiot (because lets face it, he is) because they would've known he had taken the exam so many times, since they probably remained in the same class the whole time, beignthe same age group... make any sense?

You know in one of Sarutobi's flashbacks with the 1st and the 2nd - am I right in thinking he was made Hokage then (they say something like, "Tomorrow, you will be Hokage") and he looked pretty young, like about Naruto's age or slightly older??