I'm not sure I quite agree with this. If you were in a buffet and said your kid was 12 when in reality he's 15 in order to pay the lower rate, then yes, that would be deceitful.
But I don't think there needs to be any deceit to order off the kids menu. My brother is on the autism spectrum, and except for asian food, he pretty much wants what's on the kids menu. He's not adventurous, and we don't push it. They can definitely tell he's not 12, and we're honest about that and just say, "Can we get a kid's menu for him?" It does say 12 and under, but they're okay with it and say that it happens pretty often. It doesn't hurt to ask, and in the end, they want you to have a good vacation, and if having a good vacation means your teen orders off the kids menu, is it seriously a big deal?