I found any interesting article on CommonSenseMedia.org. Apparently a study was conducted that showed that children whose parents allow them to watch 'R' rated movies have a greater likelihood of smoking. I personally don't watch 'R' rated films, for the main reason that the subject matter would more often than not be offensive to me.
And given what can be found in those films, I don't find the results of that study to be surprising. While people can claim that TV, movies or games doesn't have an impact on how a child develops, I think it does. Even if it isn't readily apparent, it can have negative effects that are more subtle. Maybe a child isn't a violent person, but on the other hand they are desentized to violent images or actions of others. So they may never grow up and act out what they've see or played, but at the same time they may be indifferent or not disapproving of what they are used to seeing.
The study does say that it may also be a factor of the parents, which I feel really is a huge part. Because if parents are allowing this, they may be allowing other permissive things to be done by their child. And in all likelihood, the parents may be the biggest reason children take wayward courses that can hurt them when they grow up.
It really helps you to appreciate how important a role a person plays as a parent, because your parenting style usually has a lasting affect on a child as they reach adulthood and beyond.