They are legal for general use, as long as they are prescribed by a doctor. It is the "off-label" loophole: once a medication is approved for a specific use doctors can prescribe it for whatever they want.
I will also ask again whether these drugs should be illegal. From my point of view, it seems logical that anybody should be able to use whatever drugs they want, so long as that drug use does not affect others. So, for example, heroin would remain illegal due to the strong negative externality imposed on a community by heroin users.
Mental performance enhancing drugs don't seem to have any negative externality associated with their use. In fact, the argument could be made that by furthering scientific discovery, they actually have a net positive effect.
I find no compelling evidence in favor of the prohibition of these drugs.
http://www.cesar.umd.edu/cesar/drugs/ritalin.asp
http://www.biopsychiatry.com/methylphenidate/ritalin.html
So, a developing child a doctor decides has ADD will experience these, and probably worse if it impacts development.
I think it comes down to the doctor's judgement, and them wanting to stay in the good graces of the DEA. The DEA has really come down on doctors that prescribe opiates lately, so I doubt very many doctors would prescribe unnecessary opiates for anxiety when a benzodiazepam or SSRI would be better served.
For steroids, they aren't really illegal, right? They're just scheduled and banned in athletic competition. I think most athletes that use steroids are probably prescribed them by a doctor.
http://en.wikipedia.org/wiki/Off-label_use (though it looks like it needs some work)