Board Thread:Naruto Discussions/@comment-34509017-20171024180450/@comment-3279282-20171024183723

Normally, statements like Zabuza's are the author's way of conveying just how brutal/strong/whatever the character is. "Zabuza thinking that Naruto was younger" would add another layer on top of the statement for no reason, since the reader would never come to the conclusion that Zabuza thinks Naruto to be younger without any further information than that.

So yeah, it's simply a contradiction, there are many of those in Naruto.