You're right that we all develop at different rates, physically and emotionally, but there's no practical way to encode those differences in the law. So they pick an arbitrary number, something that approximates an average, and make that the legal boundary. One day before it's a crime. One day after it's perfectly okay. It's not entirely rational, but it's the best that we can do. The appropriate number can be debated, but we need some number.
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Proof that even a mental midget like Zagg can lurch into the truth once in his life.
Moreover, the law is arbitrary. One year, 16 is legal. At that same time 14 may be legal in another state. Then a year later because of the media and church-ladies, 18 is the legal age.
The law and "authorities" need to stay the hell out of morality, and stick to focusing on murderers, rapists, and embezzelers.