I recommend you begin marijuana education no later than age ten, and significantly younger if possible. Middle school is often the age of initiation into marijuana use. When I spoke at one high school recently, one student asked me, “But why aren’t you at the middle school instead of here? Many here are already using.” I was recently asked by a community coalition leader to create K-5th grade flyer for THIRD graders who were bringing marijuana edibles to school! So, the earlier you start, the better. Even before a child can intellectually grasp why marijuana is bad, you can teach them that it is bad; further explanations can come later, after they’ve matured a bit.
When you do introduce the subject, don’t lecture. Try to work it briefly into a larger conversation about mental harms. You might ask, for example, what the child already knows about marijuana, correcting misconceptions before moving on. In a later conversation, you could discuss the various common names of marijuana (ganja, weed, pot, Mary Jane, MJ, reefer, 420, bud, grass, dope, wax, hash, pen, etc.), so the child realizes it all has THC and is harmful to their developing brain.
Consider bringing up the topic in the context of a larger drug education dialogue. Note marijuana’s reputation as a gateway drug: lifetime marijuana use is the #1 predictor a high school teen will have abused opioids in the past 30 days. Emphasize that modern marijuana is very strong, not the same weed prior to the year 2000, and it can cause great physical and mental harm to young people.
If the child asks if you’ve tried it, answer honestly. Be willing to discuss why, assuming you have, and why you stopped. There’s no magical age or argument that will keep your kids away from marijuana. But if you keep bringing it up from a young age right up until they leave home, ensuring they know how bad it is for them, they will listen to you more than you think.