My son was two months shy of turning three and we were questioned a few times. He is very tall for his age (95%), very mature looking and had very good speech. We were with two other kids, four and six, at the time who both had tickets. So I can see why they would think he was older. There was only one CM at the DCA gate that was a little rude about it. They never asked me to "prove" it, although I really wish I could have to the mean CM.
The child must be under three, it's not three and under unfortunately or I would have saved quite a bit of money for our up coming trip!