Doctors like to do things for good reasons. If asked whether a drug is safe or helpful, we like to be able to say not just "I took it, and I'm still alive," but rather, "10,000 patients tried this drug, and they got better faster than 10,000 who didn't take the drug, and they suffered no ill effects." But it hasn't always been easy to get that kind of information.
Before there was a Food and Drug Administration (FDA), people could say whatever they wanted about some medicine they cooked up in their basements. The FDA was formed in 1906 to make sure that foods and medicines were not contaminated with obviously dangerous substances. In 1938, the FDA also began policing companies who made false claims about the effectiveness of their products. By the 1960s, the FDA required that new drugs actually prove their safety and effectiveness before they could be sold in the United States. New laws also prevented drug companies from even advertising their products as good for any particular disease until the drug had been tested and approved by the FDA for that particular disease.
At the same time, the process for approving a certain drug for a certain disease became more and more strict. This meant more research was necessary, increasing both the delay and the expense in bringing new drugs to the public. This added expense meant that drug companies chose to spend their limited resources on the drugs most likely to sell the best: drugs that a substantial fraction of adults would take every day for lifelong conditions like diabetes, high blood pressure, and depression. This meant less money was spent researching drugs that are taken only for a short period of time, like antibiotics.
When you test a drug, you get the most reliable results by testing it on a group of people who are as similar to each other as possible. It's easiest to do this with adults. Children vary widely, from infancy through childhood through teen years. So although many new drugs have been approved since the 1960s, by the 1990s only 20% of them had been tested in children. In 1994 the FDA made rules to encourage drug companies to test their medicines in children. The courts said the FDA didn't possess the power to do that, so the "FDA Pediatric Rule" was discontinued. More recently, Congress has been trying to accomplish the same thing.
In the meantime, the bad news is that relatively few drugs have been well-tested particularly for children. The good news is that for most drugs, the fact that they work in adults seems to be good evidence that they are effective and safe for children. Everyday drugs like acetaminophen and ibuprofen are not "approved for children" under the age of 2, which is why their labels say "consult your physician" rather than giving a dose. The approval process is so difficult that few of these old drugs will ever be officially "approved for children," but decades of clinical experience shows that they can be used in children. So don't be alarmed just because a drug label says that it's not approved for children. Ask us about that drug, and we can tell you what's known about its safety in children, besides the fact that it's not officially FDA approved.