Well going by Sasukes thinking back of the time Itachi killed off the clan he looked about 8 or 9 to me. I am bad about telling how old kids are in anime years though.

I think Gai could possibly put up a better fight with Itachi. When he mentioned watching his fight that Asuma said only Gai could do that or whatever. Kakashi's mistake was thinking that Itachi's jetsu wouldn't work on him because of his Sharingon. As for Kakashi fighting Itachi Face to Face I think Gai would run around like a monkey dodging attacks getting in what he can. Now fighting face to face or running around... I think Gai would last longer.

When Itachi finished using his eyes on Kakashi you could see it wore him out by him breathing heavily and that face. So if Gai where to fight him then he could of definitely could of had a decent fight.

But lets not forget... Sasuke is going to have to kill Itachi.. same as it was destenied for Naruto fighting Gaara. With the comments that Sasuke has power that surpases Itachi.. I personally want to see Sasuke kick his ass. Oh I have a question. Kakashi said that Sharingon can't copy advance blood techinques. But if Sasuke was using sharingon on fighting or just watching Itachi fight would he be able to copy his jutsu since he is of the Uchiha clan? I would think he could.