Board Thread:Naruto Discussions/@comment-1758775-20151007125942/@comment-2140588-20151007131434

If we would take it as some sort of kindergarten, then it would be from age 3, though you can enroll in any class at any time as long as you're of suitable age (like Kushina).

Though I don't believe it was ever specified and might be different, depending on the events (Kakashi's Era and Naruto's Era).