The fuck are you talking about? There are all kinds of laws that make something legal or not based on someone's age, and they often vary by state or even locale. Hell, there's a minimum age for running for president for chryssake and it's well past any definition of adulthood.
What are you talking about? Plenty of laws discriminate based on age. Like, the minimum drinking age, the minimum voting age and the minimum age of consent
So the only thing to compare itself to is basically itself. The NZ law that it is emulating is not even a year old, so it hasn't exactly given any data on that in between generation that hasn't yet been created. I'm not saying it won't work, I'm just pointing out that this is the only really comparable law.