[1]
Itachi graduated at the top of the academy at 7
Itachi was able to use the sharingan at 8
Itachi became chuunin at 10
Itachi killed the Uchiha clan at 12, Sasuke was 7 years old.
Itachi became ANBU Captain at 13
Itachi is currently 17-18 years old.

Itachi killed his clan before he became ANBU Captain. [2]Orochimaru Left the Akatsuki 7 years ago because [3]he couldn't have Itachi's Sharingan, this means Itachi has been with the Akatsuki since he was at least 10 years of age (meaning he was a chuunin), and you would say even earlier i would presume.

Now, how does this make sense??
Kishimoto has surely pissed me off!

Referencing
Chapter 140 = [3]
Chapter 145 = [1]
Chapter 238 = [2]

If i have missed anything or something is not right, tell.