There can be adverse affects for a child who has access to age inappropriate material either online or on the TV. Little minds struggle to differentiate between fantasy and reality even if adults tell them that it is fantasy. (To be fair I think this is possibly true for adults too but at least we have the mental capacity to make choices about what we view and hopefully have some awareness of how we will be impacted.) Children are not able to make informed choices so adults have to do this for them.
I am always surprised at how little some adults seem to understand about the age ratings of films/video games, so below I have added a link that leads to more information about this topic. I feel this is very important information. In my counselling practice I see the fear and confusion that children express when they have, sometimes accidently, witnessed material that was not meant for their eyes.