This is one of the first things that I thought of when I saw that Book 4 of LOK would be set three years later.

If you were twelve when ATLA first came out, then nine year later, you’re going to be twenty-one when LOK ends. Which means that you were the same age as Aang when ATLA started and the same age as Korra when LOK finished.

And I don’t believe that this is a coincidence at all.