My brother and I were used to flying from a young age. I started at 4 months old. The only time I caused a fuss was when I was 6 months old and was returning home with my mother (we lived on an island) with a stonking ear infection. I screamed the entire 40 minute flight-not entirely unreasonably, if my later flight with an ear infection as an adult is anything to go by!
My brother was 2 1/2 when we first did a long-haul flight. We were moving to the USA, so didn't have much choice but to bring him. About the only time he made a fuss was when he was about 5 and had growing pains, which made him grisly and unpleasant to be around. In frustration, my mother eventually got out her Elisabeth Arden 8-hour cream and rubbed some of this "magic cream" on the affected area. Cue my brother immediately settling and falling asleep...bless him!
So even the most well-behaved and well brought up children (and we were both...we were frequently horrified at how other kids behaved, even when we were little) can have horrid days and there is no way for anyone else to know whether it's a regular occurrence or a one-off.
Of course it's frustrating to sit on a plane with a screaming baby, but it's also horrid to have to sit next to an adult with a personal hygiene problem, someone whose elbows seem to insist on working their way into your chair space all the time, drunk nitwits who insist on causing hassle, or my personal favourite, the bloke who thought I'd enjoy his hand on my lap every now and then. The difference is, kids really don't know any better, but adults should.