You mean 17-18 years old. He was 17-18 years old when he beat Orochimaru.
No he wasn't. Itachi was around the same age he was when he slaughtered the Uchiha when he beat Orochimaru.
He was around 13 when he slaughtered the Uchiha, guess what, he was EXACTLY the same age when he joined Akatsuki.
Why is this obvious? Because he is wearing the EXACT same ANBU outfit he wore when he slaughtered the Uchiha when during his inauguration into Akatsuki when Pein welcomes him, and Orochimaru immediately licks his lips with lust for Itachi when he sees him during said inauguration.
There is no way that you can expect us to believe Orochimaru waited 3/4 years to try and take Itachi when he looked so greedy when he saw him join Akatsuki.
Furthermore, to cement the fact the Itachi immediately went to the Akatsuki after slaughtering the clan, Tobi explained that their deal was that he would help Itachi slaughter the clan in exchange for Itachi joining Akatsuki, meaning, that Itachi had already agreed to join Akatsuki BEFORE slaughtering the clan.
Finally, I'm not sure whether it was Kisame or Sasori, but one of the Akatsuki members mentioned at the beginning of Shippuden that Orochimaru defected almost ten years ago, now remember Orochimaru defected because he tried and failed to take on Itachi. 10 years ago/almost ten years ago from that time Itachi was MOST certainly not 17 years old.