K.M.
I would use it at that age. I flew with my son when he was about 20 months and if I hadn't had it, he would have been a squiggly nightmare to deal with the entire trip.
Also, it's such a new/different situation for them, I also think the familiarity of the car seat made him more comfortable.
We flew again in March and he was about 3.5 years old, and even then he was still a little antsy - but didn't need a car seat.