Here we see Itachi, about 12 years old, utterly destroy Orochimaru in combat. In case you didn't know, Orochimaru is considered more of a monster than a ninja by even his right-hand man, Kabuto, who is himself a top-tier fighter. Orochimaru's sheer prescence was enough to paralyze Kakashi, an elite ninja at least as strong as Kabuto, and terrify Ibiki, who is not only an elite as well, but in fact a renowned torture specialist.
Here we see Itachi, not much older, making a complete fool out of Deidara, who is basically about as strong as Orochimaru, even stronger when he gets older.
Itachi, according to the current arc, is about 21 years old. Naturally, he is leaps and bounds more powerful.