Sarah Hopkins: How young is too young to discuss stranger danger with children?

Hopkins is a pedophile who unfortunately lived next to a 11 year old boy. She spent a lot of time with the child, gaining the parents trust, hanging out and going on walks around their apartment building. She eventually started showing him porn and initiating sexual activities (rape).

The boy thankfully told his mother what she was doing to him, and she was sentenced to 12 years.

In your opinion, how young is too young to couch children on "stranger danger", including that of sexual nature?