Probably sooner than you think. Unfortunately your child will be exposed to some information about drugs possibly from their friends, and possibly from someone with bad intentions.
Look for opportunities to talk with your child about the dangers of alcohol, drugs, and tobacco, like seeing some kids smoking or drinking (if not in real life, perhaps in a movie). The basis for them being lumped together is that they are all unhealthy. Why risk lung cancer, liver disease, brain damage, sickness, and early death? They are also all addicting, so why would you give away control of your life to something else? Plus, if your child is money conscious, point out that these habits are very expensive.
Now, if you smoke, drink, or do drugs yourself, it will be a hard argument to make. Your kids will do as you do, not as you say. You may need to look in the mirror first to prepare your arguments, or maybe make a change yourself. However, there is a significant difference between adults and children. Your child’s brain is still developing, so alcohol and drugs (and even tobacco) can alter her brain development. Of course, they are also illegal, so underage use could cause arrest, imprisonment, or at least a difficult record to overcome when applying to college or for a job.
When having the discussion, first make a plan and research what to say and do. Next, find out what your child knows (or thinks she knows). Make your rules clear. Explain that your first interest is to keep her safe. Explain that she is not to vape, smoke, drink alcohol, or do any form of drugs not prescribed by her doctor, and what will happen if she does. Allow questions, listen carefully, and answer them honestly. If she breaks the rules, enforce them – the sooner, the better.
One further step I recommend is rehearsing how to say, “No,” when friends offer the opportunity to “just try it.”
You can get more details and advice on handling difficult conversations about sex, drugs, and other sensitive issues, including what to say at each age, by clicking here to see your options.