itachi time line:
7 - itachi graduates at the top of the academy.
8 - able to use the sharingan.
10 - became a chuunin.
10.5 - 13 - officially became an anbu member.
13 - became anbu captain. itachi massacres the uchiha clan and leaves. sasuke is 7.
18 - itachi returns to konoha as an akatsuki member. sasuke is 13 (or close to it).

this time is based upon the fact that a year has passed since the beginning of the series. i'm not sure where i remember a year being passed but it must've happened if itachi became anbu captain at 13 and since the uchiha massacre happened 5 years ago from the beginning of the series.