4 years

An adult might speak of such a grandiose dream with a laugh. But for four-
year-old Itachi, it was precious and irreplaceable. To achieve it, he would first learn basic ninja skills at the academy, take his exams, and be formally recognized as a ninja.
This, despite the fact that the boy had still not been accepted into the
academy yet. But he wanted to become a ninja as soon as possible, so he was
training by himself.
“I’m home.” Itachi quietly slipped off his shoes in the entryway and
walked slowly down the hallway.
“How was your day?” his mother Mikoto called out to him when he passed
the kitchen. At that moment, a new life was growing in her womb.
Will it be a little brother or a little sister?
At any rate, it would be Itachi’s first sibling.
“Were you training by yourself again today?”
“Yeah.”

Itachi Shinden: Book of Bright Light Chapter 1