I think they've done a pretty decent job of setting the age, personally. Yes, there are some kids who don't have mature enough tastes or big enough appetites for adult meals to make sense at 10, but there are also kids who do have those tastes/appetite at younger ages. We see regular threads about light/picky eaters but we also see regular threads about up-aging kids into the adult plan, and that tells me Disney has pretty accurately hit the tipping point between child and adult.
My kids tastes have always leaned towards normal food fare and not the normal kids menu choices, when DD was 2 her favorite meal was chicken cesar salad. Physically they cannot eat the amount of food that the adult menus included and the reason we do not do the dining plan. Waste of food that they possibly cannot finish. Here is where I can see a junior price, 1/2 portion would make some sort of sense
I too think we will not be seeing that anytime soon as much as I would be for it