There is a lot of information online detailing many of the hazards kids and teens face online today. The breadth of the risk ranges from cyber bullying to solicitation and more. However, this realm of concern and precaution starts only when kids are online themselves (around 5 or so). What parents do beforehand matters because they not only run the risk of creating issue for their kids by their own behavior online, but they are also demonstrating to their children what proper online etiquette looks like.
For more on this topic, listen to "Is Your Child Safe in the Digital Age: How to Share without Over Sharing," a Mom Talk Radio show on which I appeared as a guest!