I was 21. I was already familiar with alcohol (since age 14), tobacco (14), cannabis (18 ), amyl nitrate / poppers (20), and opiate painkillers (19). At 21 I also first tried amphetamines, LSD, mushrooms.
Looking back now, at 33, I think it's good for me I did not discover those powerful synthetic rave drugs and psychedelics any earlier than I did (they caused me enough problems initially anyway...). Being a teenager is pretty intense at the smoothest of times, and I can only just imagine how great it must be to go through life "undrugged" and then to start dabbling on the other side of 30, or 40... the maturity reached at this stage makes the drug experience something entirely richer than it is when young... less of a rollercoaster maybe, but with more nuances. Just my 2c though. But then I did not enjoy my teenage existence all that much in the first place, and growing up fairly secluded as the country boy I was, in my early 20s at university I made up for lost time I guess... before that most of my free time with friends was spent at the village&town pub drinking beer and smoking weed in the forest.