This is a follow up to When should you start talking with your children about sex?. Is it better for parents to preemptively educate children about taboo subjects such as drugs, alcohol, sex, or profanity before they are exposed it in school? Or is it better to let them discover them on their own through their friends?
These things you list are a part of our lives, either because we use/do them or other people we know or see use/do.
Discovering things through friends is rarely a good idea. That's how myths are spread, "you can't get pregnant the first time", "masturbation will make you impotent", ... "drugs are good". You should give your children the important information yourself.
Drugs, alcohol and other substances which alter our behavior(energy drinks, pepsi, coffee, antihistamines, painkillers, etc. etc.) should in my opinion be discussed with kids. I think that a child around 10 is perfectly capable of understanding how alcohol or drugs work and what they can cause. Perhaps even earlier. But don't go into much details about drugs until some time later, I think.
Children should definitely know about sex. Girls should know "the general idea" (penetration, sperm, egg, fertilization) before they begin menstruating, I think. Perhaps have the "sex talk" along with the "period" talk. And since it's not obvious when will that happen, 9-10 yo is a good age. For boys - it can be a year or two later. Watch the pajamas for the new kind of night wetting.
If your child asks you "uncomfortable" sex or other adult questions on his/her own, use this as an opportunity to have a talk about those subjects. Say the truth, though, possibly, skip some age-inappropriate facts. Use common sense.
Profanity is probably a part of their lives already. Even if you don't swear, they do hear it on the street. It should be addressed very early, at 3 years of age? A kid will catch the swear words very quickly and easily. Let them know that they shouldn't be used (...too much).
Perhaps, as in many things in life, it is a good idea to limit the information given to the necessary minimum and let the unimportant rest be learned through friends or Internet. As much as necessary, as little as possible.
Who do you think will be more likely to provide answers in a way that you would want your children to learn about drugs and sex, you as the children's parents or their peers (whose parent's might also evade the subject)? I don't know about you, but I trust my judgment on and knowledge about these subjects more than those of other children.
Of course, given your question is about "children", this leaves a lot of wiggle room. Are we talking about a 5 year old who noticed her uncle acting funny at his dad's birthday party or are we talking about a teenager who might have access to drugs and sex (porn) if they want to? As a general rule, I wait for children to raise a question. And I make sure I only answer the child's exact question, and do not evade the question nor pile knowledge on the child it hadn't asked for.
That is, if a 2yo asks where babies come from, "they grow in their mummy's tummy" is likely to be exhaustive. A 4yo will likely ask how they get into their mother's belly, or how they start growing. My first take on this would be "it takes a woman and a man being in love for a baby to get started". If the child wants to know more, it will follow up with a question. If the child does not want to know more, the description of the mechanism of sex might be too much for its age/maturity.
IME, if you do this from the moment they ask their first question, you do not need to worry about when you should teach what subject to them: You just let them guide you through their education by having them ask you. Of course, reality isn't as smooth as this sounds. Still, this seems a lot better than wondering year after year when to finally breach the news to them that adults have sex and that all their peers are the results of this.
For example, my oldest got interested in the details of making babies around the age of 8. Her next sibling just came to listen because he was interested, but when it got too detailed, he just went back to playing because he got bored. (He could do so because I wasn't talking to him.) OTOH, I have a ten year old now who had had a look at the book we bought for his oldest sister for about three times in the last two years, and always got bored by the subject. He's just not interested enough. (He does, of course, know the basics: why he's built different than his sisters, what's that for, etc. – he just isn't interested in any more detailed knowledge about love and sex.)
Most of the other advice in this topic is excellent but a small item to add:
This shouldn't need to be said but there seem to be a lot of parents in this world who prefer to try to control their children through lies. If you lie about sex, drugs and alcohol once they catch you in one lie they won't trust your other advice on the matter.
Sitting children down to "tell them the facts of life" or educate them about drugs etc is a stressful proposition for parents and children alike; it often feels confrontational and uncomfortable for all involved. A proactive education is very important in some areas and a reactive approach (responding to their questions) is better in others but which occurs when depends on your circumstances...
In sex education it is vital to ensure children understand their intimate privacy and what is appropriate / inappropriate contact long before they have any need to know the mechanics of reproduction. It is not something that we ought to wait for children to ask about as they may not do so, especially as most abuse cases happen at home or with people victims know well.
Other subjects are less clear-cut like Alcohol... Parents who drink responsibly teach children what the effects are by example. They show what the changes in behaviour are and that if you overdo it there are consequences (even if only a bit of a hangover.) Parents who don't drink or hide it away need to consciously make an effort to bring it up to ensure that it doesn't remain some taboo, but is considered something to be treated with respect.