I'm afraid to let my children get into relationships(i.e. girl-boyfriendships) too early just in case it did them no good. At what age would it be safe to allow our children this liberty?
To preface, my answer is based on a personal belief in abstinence before marriage, and on guidance I received from my own parents and church leaders when I was a teen on how to structure relationships in order to make that goal attainable. It worked for me, and I plan to encourage my own children along the same lines when they are older. My intent isn't to start a debate on the merits of abstinence, merely to answer the original question from the point of view of someone with that goal in mind for their children. It might also be useful for people who only feel abstinence is important up to a certain age. If such a goal offends you or you find it anachronistic, feel free to ignore my answer.
Overly restrictive? Maybe. I chafed at it myself a few times, but then I realized I was having dates more often, with a larger variety of activities that most of my more "liberated" friends. Ironically, because I was specifically avoiding looking for something serious at first, I didn't worry about going on a date that might not work out. My friends were more reluctant to ask someone out because of increased expectations.
In contrast with the other answerers, I think that it's reasonable and healthy to restrict a child's ability to go out on dates and engage in romantic relationships in accordance with their age. You have to strike a balance: romance, dating and sex are dangerous and difficult to get right, but practically speaking you can only learn how to successfully negotiate them with a certain amount of trial-and-error. You need to help your child by making an informed judgement of their ability to make good decisions and handle heartbreak.
That said, here are some general guidelines:
Of course there are exceptions to all of this. I know a couple that began dating when they were 13, and dated continuously with a few breaks until they were 18, at which point they married.
Why would you want to restrict your child's attempts at a romantic relationship? It is not alcohol, or smoking: there is no fundamentally negative effect from dating.
If you are worried about your child becoming sexually active, trying to restrict it won't help much there, either. Such attempts will likely have the opposite effect: teenagers can and do rebel.
As for the possibility of a broken heart - you can't have any affect on it at all. What is important is that your child has your support and trust, and trying to restrict their (totally normal) forays into puppy love or romance won't earn you their trust or encourage them to come to you for support.
Nothing says "I think I love you" like "my parents don't want us to be together."
I'm not certain what you mean by "just in case it did them no good." Not every relationship is going to work, and this sounds dangerously close to setting the expectation that you should only enter into a relationship if it will result in long-term commitment.
Early dating should be a learning experience. It is just as important to learn how to identify and get out of a relationship that isn't working as it is to learn what makes a relationship work.
As Nikita mentioned, attempting to restrict your child's romantic relationships will most likely not work. In fact, attempts like that all too frequently backfire. At best, you're fighting against peer pressure and your child's desire to exert control over their life. At worst, you're fighting all that plus hormones. Your best bet is to educate. Make sure they understand the pitfalls of dating, and give advice on how to deal with various situations (how to show appreciation, be willing to listen, how to apologize, how to draw boundaries if necessary, etc.).
I think dating is generally a tough topic for parents because it is one of the biggest signs of the approaching end of childhood. It reminds us that we won't be the most important people in their lives forever, and that they are becoming more and more capable of looking after themselves and making their own decisions. It is very easy to lose sight of the fact that that is the ultimate goal of parenting.
When is it safe? Relationships and dating are never safe. Even at 25 it's like running headlong into a minefield of extreme emotion and high passion. That's what makes it great, after all.
Now that you've taken the bubble-wrap off, it's time for an opinion from a lefty. Coincidentally, science and statistics have shown that these methods actually work better at delaying pregnancy and sexual initiation.
First, both genders need to know about sex. No, of course they're not going to be having sex right now! (what? You're asking this question when they're 16? Oh shit, too late!) They should have already gotten the biology-of-reproduction and the "why adults like it" bits years before they start dating. By now, they should have all that down pat. If they haven't gotten it from you by the time they're 11 or 12, then they've surely learned something on the playground, and now it's an uphill battle because they don't want to listen to practically anything you say anymore. Unless you're really lucky.
Both genders also need access to birth control, or in the case of girls, be on birth control, "just in case". Think of it like making your 4 year old wear her bike helmet, even though she still has training wheels on and she's still not riding faster than she can walk. Good habits are easier to make than bad habits are to break. And besides, people don't always plan things a month in advance. Or plan things at all.
For girls, these are prerequisites:
Now you get to let them loose and catch them when they fail miserably. That's generally your job once they're teenagers anyway.
Coincidentally, abstinence before marriage is a recipe for marital disaster that starts young and ends early (hopefully before kids, but often and even usually, not). For god's sake, find out what they're really like before you get married.
I can be considered a very oddball example, but this is just my view, feel free to ignore it. I had absolutely no rules on dating, but have decided on them myself. I met my soulmate when I was 16. Now if I had someone impose rules on me then I would have never met that special someone. At the same time I have enforced abstinence before marriage. I felt that not being physically attached to someone can help keep a clear mind when it comes to thinking about the big commitment.
I do not enforce any rules on my kids. Life is a test, you will never learn by following someone's advice. We are all different people, and what works for some might not work for others. Additionally I think that if I enforce rules on my children, then they will still try to do things and be secretive about it. I would rather be my child's best friend and trusted adviser, than a tyrant who is trying to prevent him from (in his eyes and opinion) having fun. I did notice that allowing a child to try whatever they want and just giving advice here and there is better rather than forcing them to do something. Additionally advices are usually followed, whereas rules are made to be broken.
So my two cents would be not saying anything, and only speaking when spoken too. Trust me they will come to you asking for help when you have an open and helpful mind waiting for them rather than a judgmental mind.
I think a good solution is to not allow or encourage early dates (or seeing forbidden movies, or smocking cigarettes, or sneaking out of home at night), and let these activites be on the "transgression" side of the life of the kids.
In these times of so-called transparency, I might be even more old-fashioned than the abstinence proposing answers, because I don't think it is possible, and not even desirable, to be fully transparent with our kids.
They want to have their secrets, so just let them believe you do not know about what they did last night.
Another consequence of this controlled semi-opacity: I will lie when my kids will ask me if I ever tried drugs.
So if my boy comes back from a romantic walk in the fields at 2am, I'll not be waiting for him with a loaded gun, nor will I offer him a last beer. I'll sleep (on one ear, and be envious of him). And if he get caught, I'll tell him it is not ok, even if I think it actually is ok.