True, Itachi may have been an anbu captain at age 13, but Kakashi and Itachi had the same class rankings. Yet, Kakashi had alot to deal with when he was young , like his father commiting suicide and his team getting anbushed, leading up to his current sharingan. They never tell you how old Kakashi was when he became an Anbu, but he was definetly a teenager. So, in a way I agree with Saisano, but I also agree that it might be a tie. Still, I can't see Itachi winning this in any situation.
in my experience, ANBU Itachi would defeat ANBU Kakashi because ANBU Itachi has more powerful jutsus like Tsukyomi and Fire Style: Fire Ball Jutsu, while ANBU Kakashi has the basic jutsus of Summoning Earth Style: Fanged Pursuit Jutsu and Lightning Blade. And lets not forget, normal Itachi got Kakashi in the Mangekyo Sharingan, when they first fought in the series.