I am a rural family physician and my recommendation: before age 9. Before third grade.
WHY? Your eyes are popping out of your head in horror, but my recommendation comes from surveying my patients. For years.
The biggest drug killer is tobacco. However, it takes 30 years to kill people. It is very effective at taking twenty years off someone's life, destroying their lungs, causing lung cancer, heart disease, mouth cancer, breast cancer, uterine cancer, stomach cancer, emphysema, heart disease....
I ask older smokers what age they started smoking. This is informal. This is not scientific. But most of my male older smokers say that they first tried cigarettes at age 9. I think parents need to be talking to their children about cigarettes by age 9.
And then start talking about alcohol and illegal drugs and the terrible dangers of pills.
"My innocent child would never...." Unfortunately my daughter said that as a senior in high school in our small town, there were 4-5 kids out of the 120+ that were not trying alcohol and marijuana. But there are kids trying far worse substances. We have methamphetamines here, and heroin, and pain pills sold on the street.
The perception that pills are safe is wrong too. Heroin is made from the opium poppy and it's rather an expensive process, not to mention illegal and has to be imported from dangerous places. But teens take oxycodone and hydrocodone, bought on the street, to get high. And now drug sellers are making FAKE oxycodone and hydrocodone and selling that on the street. It contains fentanyl, which is much much stronger. If the dealer gets the mix wrong, the buyer can overdose and die.
Talk to your children young! NEVER take a pill from a friend, never take someone else's medicine, never take a pill to party! YOU COULD DIE! And if you have a friend that is not making sense, that you can't wake up, DON'T LEAVE THEM! Call an ambulance. Your friend may have used something illegal, and may not want you to call an ambulance. But if you think they are too sleepy..... don't take a chance. People can get so sleepy, so sedated, that they stop breathing.
And parents, you are the ones that have to set a good example. Don't drink alcohol every night. Don't use pot every night. Take as few pills as possible. Pills aren't necessarily safe because they are "supplements" or "natural" -- hey, opium and heroin are plant based! Stop using tobacco and if you have a hard time doing it, tell your children you are struggling. It takes an average of eight tries to quit smoking. Get help.
Talk to your children about addiction young... so that they can avoid it.
I mean hey, if they can babysit at age 11: https://www.reference.com/government-politics/legal-age-babysit-california-d509b2dfd5c1a47d
The goood news is overall use in teens in decline: https://www.drugabuse.gov/related-topics/trends-statistics/infographics/monitoring-future-2016-survey-results