Why over 18? Younger seldom seems to matter to the sellers.
Anyway Alexander Shulgin and his wife would rate as personal heroes of mine and drugs were very much his/their thing. You can read about the guy elsewhere in far more depth but the short summary.
In organic chemistry the dominant concept would have to be that of the functional group, that being the part of the chemical that does the reactions for most things while the rest somewhat stands around looking pretty. Looking pretty is a bit of simplification because if you add more to it, parts out for other things (put an O-H there where there might have been a H before, NH3, there are many options so I will leave it there) it can vary what happens quite dramatically (
one pill makes you larger, one pill makes you small, the ones that mother gives you don't do anything at all is more accurate than you might imagine). Prior to making it there is not the most you can do to know what it does without testing it for real, at least prior to supercomputers a few years ago and they are not dominant in research yet.
This guy made hundreds of variations on the compounds, detailed how to make it, how to make the intermediate stages, what the yields were. He is credited with making the manufacture method used by most MDMA/MD?? dealers, some debate whether he discovered 2CB but he certainly brought it to the fore/is the public creator of it if not, on DMT his methods are probably those used and so on, many methods are also geared so people without so much chemistry skills can pull it off. Oh and after he made these compounds he did a bioassay on them, which is to say he tried it, tried higher doses and note the effects. Again hundreds of compounds he tried in two classes of chemicals, those being phenethylamines (the family of MDMA and 2CB) and tryptamines (the T in DMT). He then gave it all away for free (ish, there are two books called PIHKAL and TIHKAL and the chemistry parts are free where the first parts being a story/diary sort of thing detailing it all. If you are more familiar with chemistry much of this happened in the 80s and early 90s (the books being published in 91 and 97 respectively) and I believe for much of it IR spectroscopy was about as high end as it got for chemical ID. He died in 2014 at the age of 88.
For some all that would make him villain of the century but from where I sit he ranks as a legend.
A worse variation on this method is what you would have seen in "legal highs" where base compounds were tweaked to make other things that were not illegal and then sold without much testing. This then led to awful laws like those trying to ban all things that get you high in various places.
That said I am a boring bastard and have not found a drug I care for (and that includes alcohol and caffeine).
http://gbatemp.net/threads/nootropics-smart-drugs-thoughts-and-opinions.399108/ is about as close as things get and there seems to be little there right now, and psychedelics don't lack appeal but not enough to bother. I have no problems with others going in for it though.
Though if intense sleep deprivation counts I will go in for that.