The most likely answer is that he was only about 20 years old. He had been born in the year of the snake, which correlates to around 10 years before the Uchiha massacre. This means that Itachi was born just before or during the Third Great Shinobi World War, which would make him about 20 years old when the war ended in a stalemate.
It should be noted that this is all speculation based on evidence and circumstantial evidence. For example, there is no record of Itachi ever being born, so it’s possible he was never even born at all. Also, while we know that Sasuke killed Orochimaru, we don’t know if Itachi did or not (if he did then it means Sasuke wasn’t really alone when he killed him).
Last modified: September 19, 2022