G/f's step brother has flown all of his from about 3 months from Europe to Australia. The long flight aside, they can do it. The basinettes are for up to 6 months old that hook onto the bulk heads so that is another indication of age people fly with young ones at.

As long as you and the wife can cope with a kid in the air (screaming especially if their ears don't pop), go for it.

Edit - Just made sense of Dojj's post and some of mine is covered in that.