If someone knows something exists, they will be more compelled to investigate if they don't know it exists. They will find out it exists anyway through their friends - this is inevitable. Therefore, if you keep telling the curious person, "no", they'll be compelled to investigate the issue for themselves to see why they can't get it.
People since they were kids do not like to be told not to do this and that. Nevertheless they should be disciplined accordingly. But there is a certain age where people prefer to do whatever they please. But once they achieved what they were held back to do they no longer have that need to do it.
People will establish something as taboo because it is considered wrong. When something is censored, people recognize it as something they shouldn't know about. This establishes our morals and code of conduct, and keeps individuals that believe in these morals from viewing something that is censored. We protect ourselves from doing things we think are wrong.