This reasoning never made sense to me. What the hell are these kids' parents doing and why is this something that needs to inconvenience everyone else? If lazy parents don't want to monitor their children while they spend all day on their ipads, that's their problem--it shouldn't be made mine.
Kids will find a way to sneak around their parents every time, esp. if their friends (or a groomer) introduce them to something "cool". Active parental monitoring alone isn't really a solution.
Liquor stores shouldn't have to check ID.
Parents should be aware of where their kids are at all times and physically prevent them from entering liquor stores.