This  helpful resource guide from Partnership for Drug Free Kids gives excellent advice on how to start the conversation at any age.  From their website:  "Conversations are one of the most powerful tools parents can use to connect with — and protect — their kids. But, when tackling some of life’s tougher topics, especially those about drugs and alcohol, just figuring out what to say can be a challenge. We’ve provided scenarios and scripts below on what to say to your child, no matter their age."