Violent crime is a basic thing. Even with highly subjective morality, you simply don't find any societies that promote violent crime within their citizenries. None.
We can call it "wrong" or "bad" or "not condusive to a functioning society" or "Bob"—whatever makes you happy, but there's a global, fundamental stance against violent crime. Thus, everyone everywhere knows from an extremely young age.
I have seen many games and movies that promote violent crime directly. But violence is a reaction, not the sum of a cognitive calculation.
The amount of crime is a stable number. It isn't that random. This means that crime prevention methods can be seen in statistics.
Countries with few or no systems to prevent violent crimes from happening before they happen, such as those who believe a written law is all it takes, tend to have a lot more violent crimes than the rest of the world.
That's a decision. In most cases where it's decided that the society is best served by methods that are proven to not work, the decision is taken by ideas such as "the academia have a liberal agenda" or other identity-serving biases.
This would be fine for "higher level" issues, say cheating on your taxes, but really isn't applicable to something as basic as violent crime.
Yep, arbitrary decision based on cultural mores. Kinda make the argument "Our arbitrary decision based on our cultural mores is right and your arbitrary decision based on completely different cultural mores is barbaric" seem a little silly, doesn't it?
OK, but even you said "18 is usually an age", so you're already agreeing that the line is fuzzy and varies from culture to culture. So to then turn around and get snotty because someone else draws that fuzzy line in a different place is just plain silly.
The ages are both culture-bound and not. Like I said, the setting is a framework for cognitive development although cognitive development is a system within itself.
Abstract thinking begins at around 12 in western cultures, perhaps as a consequence for being educated in a culture that promotes thinking and give a child the tools (education spoonfeds experience). This mind continue to grow but will ultimately work within the cultural frame which just so happens usually means "school, family, close friends" up to the age of ~18.
Research done on non-western countries have shown that some skills develops faster and other skills slower depending on cultural framework and what kind of insight is promoted by the culture.
But the mind is the system that interacts with this environment. Some people grasps things faster than others. Some people interpret the information in radically different ways (disorders). Some are forced to early on skip out on the system. Some are promoted to always be within the system. Some are encouraged to think outside the box. But all of these variations eventually end up in a deterministic and solid value based on probability theory that keeps itself solidly attached to how the society as a whole treats people within a certain age.
Ideas such as "people who are 18 are adults" thus become very important not only for juridical thinking but also cognitive development.
Which is also why it's important for a culture to be consistent. If the system treats people below the age of 18 as non-adults they also have to be treated as non-adults through and through, because treating a 18 year-old as a non-adult have a huge impact on how their cognition tick.
I once heard an American anthropologist speak about how one of the greatest problems with the US was the fuzziness around what it means to be an adult. Many European countries have a very solid transition in which people are told that at a specific day they will be treated as an adult and they have to act accordingly. This gives the mind a new frame to work with.
So yeah, its important to have a special day for it and it might actually have more real impact to it than it might seem.
