This would be relevant if the Jetstar actually refers to unaccompanied minors, but it doesn't. As an experienced frequent flyer you are implying this based on the fact that other airlines do mention unacompanied minors, but not relevant to Jetstar as they do not seek to relate "children" and "minors". In fact their rules seem to specifically differ to other airline rules so any attempt to suggest they should be assessed as similar would seem to be misplaced.
As an experienced frequent flyer we may all know that this is a way to charge more for flights however I think you will find legally people ARE entitled to rely on Jetstar's only publically stated position on what is a child. The legal definition of "knowledge" about such things doesnt refer to people like us who have extensive knowldege of such matters, but the average man on the street, for most of these the only definition they would be aware of (unless Jetstar made it obvious) would be this one.