It can start as soon as kids begin to go online to sites that offer interaction (chatting, for example) or getting cell phones with texting capabilities. I don't know if it has more of an impact at any age. I think it impacts kids differently at different ages, but the important thing is stressing how to use online communications respectfully, responsibly, and within the limits you set.
Information provided by Caroline Knorr, Parenting Editor of Common Sense Media. For more tips about teaching your kids to be safe online, visit Common Sense Media