I have a 14-year-old daughter and an 11-year-old son who watch TV and explore the Net. The amounts and levels of violence and sexuality they are exposed to are mind boggling. On the one hand, I understand that their generation's value systems cannot be compared to ours, as ours were not comparable to our parents'. However, I do think that until a certain mature age, kids should be protected from overt sexuality and violence, until they reach a level of maturity where they are then able to process this input without it distorting their values. I am confused as to when they should be allowed to explore these media freely and how I can censor their exposure with their cooperation.
Your concern is a real one. Kids do not have the ability to absorb the kind of input you describe without it having a distorting effect on the way they view the world and themselves. It is our responsibility to protect them from sensory abuse just as it is our duty to protect them from physical harm. Depending on your relationship with your kids, you may be able to explain your action rationally.
Eleven and 14-year-olds generally have the ability to understand the logic behind not allowing them to view TV and roam around the Net unhindered by any form of control. You may be able to trust them to abide by the strict guidelines you set them, e.g. listing the programs they may or may not watch. If you think this method may not work, there are programs available to block reception according to your guidelines. As to the timing, this is a very personal thing. I am sure that you will know when to loosen the reins that bind your kids to you in this as in other ways.