I recently boarded a DTW/LAX A320 flight near the very beginning of boarding. I put my stuff away at my exit row seat, and went to the back to use the lav. They were both locked! I couldn’t imagine how 2 people could already be in there, as I boarded so early! I asked the FA if they were occupied, and she said, no, they just don’t flush til you hit 16,000 feet! I asked her what I should do, and was told, to go out into the terminal and use the restroom there. (Yeah, right! Swim upstream, get off the plane and start all over again?) I didn’t, of course, but any ideas as to what that was all about? I've used the lav on the ground MANY, MANY times! People may need to do something other than use the toilet! I go in there to take medicine, sometimes!