Sooo, hi. Sooo, weeks ago one night me and my friend was texting, and were talking about this book, and she pointed out. That Itachi was 13 when he joined the ANBU, and when he left the village. So, in the flashback's let's say hm.... At first I said mmm, he left the village at 18. BUT I thought, why did he leave at 18?

Answer~

He left at 18, cause he wanted to stay in the village for a bit longer, and the day him and Obito decided to destroy the Uchiha clan, Itachi told him that he wanted to spend a few more years with the one he loved before leaving. So, Obito being a bit sensitive (I don't know what I'm typing any more) gave Itachi 5 more years, until the massacre happened. (Sasuke would be 12 by then sooooo)

So, that's it yea I explained enough, hope it's not too confusing if it is just comment or message me and I'll explain a bit more.