Before 18 (or 21, or 16) it's debatable that humans have the capacity to self-control, or make their own decisions understanding the effect on themselves and those surrounding them. I'm not saying that they're completely oblivious, but there is a reason why the law is not applied in the same way for children (non-adults).

Now, it's a mix of societal and economical (and more) that's making people eat a certain way. Changing your eating behavior can be difficult, especially if you've eaten a certain way for the past 16-18 years.

I'm not saying it's Walmart's fault, but it's also not just an individual willingness to change that's in play here. The United States clearly has a problem of obesity, which is related to Type 2 diabetes.