I find your statement to be pretty offensive to so called "children". You are implying that they are completely helpless and have no ability to make decisions with consequences.
No, they have
less ability to realize consequences, and more to lose because of undeveloped bodies and minds. You seem to imply older teenagers but what about an 11 year old? If I was to lay out a line of coke and say "snort it", you think she would know the consequences? If she was parented right maybe but not every kid.
In my book, if they can figure out how to use the internet, tor encryption, bitcoin, order something through SR, all without it being detected by anyone who cares to stop them, they are certainly mature enough to comprehend the consequences. Please consider that you are turning kids into bubble boy (if you've seen that movie).
Today, Tor and Bitcoin might be pretty hard for youngsters to get a grasp on, but in the future? Let's dream for a minute, and say in the future phones can come with Tor installed, and bitcoin is a common payment method. If a child, 8,9,10,... gets recommended "Silk Road" by his older friends and he's able to buy some stuff that makes him feel awesome in the blink of an eye, is he able to see or weigh the consequences?
Children's freedom is definitely important to me, and I do think western countries are learning to control their kids too much, but drugs are not something to be introducing to a young developing human being. I can't tell you if 18 is the "adult" age but at least by then you have the ability to think for yourself a lot better than being young, and everything learned until teenage years is basically parents/teachers ideas handed down.