None! Banning is for wussies!
But joking aside; Banning is the bane of human intelligence. It is one individual putting himself or herself above all other minds, saying that their decision is final, and that no one other person can disagree with them. Bans have stemmed from political to religious to purely subjective reasons.
Saying that young minds are impressionable is true to a certain extent; the younger the mind, the more chance of absorption without cognitive meanings attached: no, I don't mean kids are stupid, I mean there are certain biological targets that the body has to reach first before giving certain synapses the get go. But it is also true that when allowed, children ask questions on the world around them, what they read, and what they don't understand. Being honest with them allows them to form their own opinions of things without discounting others. This is how I was brought up; mum and dad never stopped me from reading anything, and only made sure to explain to me that opinions were always relative, and that right and wrong cannot be judged when one is apart from the situation, text or creed involved. Having opinions in life is not a bad thing, but actually a necessity (your brain has to filter out all the noise in some way, and we all think some things are good or bad depending on our upbrining), but ultimately, imposing what one believes on someone else is the greatest sin against human kind.
So no. Banning to me means oppression of the gravest sort; it's akin to keeping people illiterate, because people who can read and write can understand, and travel farther with their mind. I'm all for banning bans ... but that's just me :)