Note that consumer marketing, especially with regard to entertainment and recreation, vacations and such, tends to recognize serving children as a loss-leader: If you make the cost of bringing children along less, that tends to break through the resistance to making the purchase that is often thicker with parents worried about feeding more mouths than DINKs would have to concern themselves with. ... So, effectively, a overage patron ordering a menu item intended to be a loss-leader is getting more than the eatery was strictly intending to offer.
There should not be any age limit. I'm in my 40's and not a small girl and I often order off the kids menu at restaurants. I do so because either I want a small portion or because of my own food issues I'm happier with something simple. If they told me I could not I would leave. And I have. What they need to do is do away with "kids menu" and just have some smaller simpler items on the regular menu or offer half portions for those who want them.
However, that would destroy the kids' menu's ability to serve as an effective loss-leader for restaurants.
Even beyond that, I suspect that folks (
especially parents of young children) would be complaining in droves if kids meals were really just small and simpler versions of regular menu items, priced in the same way as the regular menu items (i.e., not as a loss-leader). The cost of food ingredients is a very small part of the overall budget of a restaurant. Halve the portion, but keep all other aspects of the pricing the same, and you reduce the price by about 4%-5% - now
that's something to complain about, paying so little less, but getting half as much food. The pittance nature of the difference in price between a small steak and a large steak has prompted some WDW restaurants to eliminate different size cuts of steak.
What are you paying for when you eat a restaurant meal at WDW? The labor, especially. You're paying for people to greet you, seat you, server you, cook for you, and clean for you. You're paying for the facility itself, the cost of building it, maintaining it, furnishing it with tables, chairs, dishes, silverware, etc. All that is obvious. What's not obvious is that you're also paying the cost of customer acquisition and you're paying an extra premium for the convenience.
That is one thing that makes WDW restaurants rather unique - that so much of what you pay is attributable to an intangible. And how much food is on the plate has no impact on how much an adult appreciates being able to sit down and enjoy a nice meal, without the rigmarole of leaving the park midday, getting to your car (assuming you have rented one), driving to someplace else, having your meal, and then coming back.
However I'll tell you that your use of 'mooch' is offensive and argumentative.
I agree. I've never seen that word used in any substantially respectable way. Folks who feel that adults should not order off the kids' menu, for whatever reason, should call the practice exploitative or abusive (like I do), but not "mooching".

We're adults here: Let's use intelligent language, rather than childish language.