We stopped taking the car seat when DS turned 3.
Before he was two, we never bought him a seat. He either sat in our lap, or we were able to bring our car seat on the plane because there was an extra seat.
From 2 to 3 we used the seat when we traveled, but by the time he turned 3, it was way more hassle then it was worth.
I can't imagine using a car seat for an almost 6 year old on a plane. There'd be no room for the kid's legs.