I have been to many restaurants that restrict their children's menu to children - it is clearly stated on the menu. But, I do live in FL where there may be more adults with smaller appetites (ie, the elderly) who would order a children's portion. The restaurant's usually get around it by offering early bird specials and 1/2 portions, so that they still attract seniors. And many have a plate charge for sharing, because it's so common.
As Monty mentioned, the children's menu is often priced at a loss that restaurants offer based on the assumption that the adults in the group will order something at full price. If they allowed anyone the option of ordering a child's meal, they would lose money.
But I have found that most restaurants - especially at WDW - will allow an older child the option IF (s)he is with an adult who is ordering a full priced meal.
As long as the policy is clearly written, an establishment can make any rule as long as it does not violate law. And some policies can be changed at will, such as holding an item on layaway. As I discovered a long time ago.