For as long as I can remember. I mean...I was really small when they figured it out. I think because Jinora airbent first. And then, when we were kids, she used to play with me a lot...I mean, she still does. Did. But back then, she bent breezes at me, I think 'cause it made me laugh, and one day I bent it back at her.

Also, airbenders sneeze really obvious.