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Okay, I'll bite.
Your point about the age of consent being somewhat arbitrary is, contrary to your protestations, neither new nor original. The law draws all kinds of bright lines demarcating when one thing becomes another. That's what the law has to do.
To use your point, I can't go into the Army and be killed in Iraq when I'm 17 either. On the day I turn 18, I can enlist. Replace 17/18 with 20/21, and I can drink alcohol. The cops may not pull me over if I'm speeding modestly, but they most certainly will if I'm going 15-20 miles over the speed limit.
Your argument could just as well be used to lower the age of consent for sexual activity, which you will find to be the case in many European countries. There are obviously some 16-year-olds who are more mature than some 18-year-olds - and for that matter, maybe even some 80-year-olds - but the point is irrelevant. The law can't be written for individual idiosyncracies. For better or worse, our society has decided that people are (except for drinking) adults at age 18, and makes them free agents at that age. In most times and places in the vast sweep of human history, people were effectively adults at far younger ages than 18. To imply that people can't make rational choices at that age is profoundly condescending to intelligent young people everywhere.
We get it. Porn offends your moral sensibilities. The answer to that is obvious: you don't have to consume it. But the poorly reasoned diatribes are really rather ridiculous, and just a thinly veiled attempt to try to assert the superiority of your own moral sense over that of others by force of law. It's authoritarianism, whether you admit it or not.