Is Itachi Naruto's blood brother?
Naruto has no physical relation to Itachi as he's the son of an Uzumaki and an Namikaze.
Why did Itachi join Akatsuki?
Itachi Joined Akatsuki To
Serve As A Spy For Hidden Leaf Village & to keep his brother safe in the village. Itachi also threatened to reveal all of Konoha's confidential information to foreign forces should Danzō bring harm to Sasuke.
How old was Itachi when he joined the Akatsuki?
8 Itachi Uchiha joined the Akatsuki at the age of
14 & died at the age of 21 during his battle with Sasuke. An Uchiha prodigy and elder brother of Sasuke, Itachi joined the Akatsuki at the age of 14.