The ANBU, meaning "dark side" is actually short for Ansatsu Senjutsu Tokushu Butai (literally Assassination Tactics Special Military Force). The ANBU work directly under the leadership of the Hokage. This specialized ninja battalion generally work in groups of four to five. They are responsible for assassinations and missions requiring highly trained ninjas.
You don't need to be a Chuunin, Jounin, or whatever. It's just a name.
If you wanna be an ANBU, it depends on how strong you are. Like Itachi became one at the age of 13. It isn't even known if Itachi became an official Jounin before wiping out his clan.
The ANBU come to the ninja, and ask to join their organization. It's basically like Special Ops, and they're just a special part of the Ninja force.
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