As Graham and Terpe mentioned <24 months = infant which is normally 10% fare. 24 Months+ and its a child fare, which often only 10 or 20% less then an adults fare. We've just flown recently with my 23 month old son and 5 year old daughter, and purposely chose seats at that the back hoping for an empty row, and got lucky, so the kids got to lay out and sleep.
