Dr. Annie Zimmer, Boys Town Pediatrics
It is still good for your child, especially as they are leaving the house, preschool or kindergarten age, to learn safety rules with adults that aren't known to the family. So your child should know that adults shouldn't need to ask them for help. Adults shouldn't give them food or treats if they don't know them. They shouldn't ask them to come into the house or get into their car. And if any adult is making your child feel uncomfortable your child should know that they can always tell somebody no and if they are feeling scared by an adult they should run away and yell and then tell somebody about the experience that they had. It's probably best that parents just keep a good eye on their children and not putting them in environments where they might be exposed to a stranger and something bad could happen. As they get older they can understand good and bad strangers and adults better.
I would start this at four or five, just some simple stuff. You don't have to get into it to the point that it makes them nervous, but start talking about who strangers are. A stranger is just somebody who your family doesn't know. It's always ok to reassure your child that it's ok to talk to somebody new, especially if you are there with them, but if they are by themselves and an adult approaches them, then they should use caution and treat them like a stranger.
Stranger Danger is a very important topic that parents should discuss with their children. Dr. Annie Zimmer, pediatrician with Boys Town Pediatrics, offers advice and ideas on how to discuss this safety topic with your kids.