for me...
14 - alcohol
18 - ecstasy and assorted other "hard" drugs
19 - marijuana
i think the age i tried it at was fine, as i was eighteen, and had finished high school. if i was within a different social circle of friends i might have even preferred to wait even longer (god knows i would have done better at uni then!), but i don't think my drug use is particularly irresponsible (both in health terms and in the way it effects the other parts of my life), so it doesn't trouble me too much.
for me, "hard drugs" (mdma, meth, etc.) are probably not something that i think most high-schoolers should be messing with, as i just think it's too much to cope with. in hindsight, year twelve was an absolute piece of piss, but it's obviously the mental pressure and expectations placed on you that are the most difficult things - not the work itself. thus, fucking around with drugs at this time is probably just going to make things even more difficult.
that said, i'm sure there are plenty of well-adjusted and intelligent people who could cope with it just fine. i guess i have to remember that in year twelve i was still going out *every* weekend and drinking half/whole bottle of bourbon everytime...i just think that's easier to recover from (brain-wise) than taking pills every other week.
point is - i think that health wise you're best waiting 'til at *LEAST* eighteen (to allow your body to complete - or close enough to - its growth); psychologically (in want of a better word), i think the same sort of age - after year twelve - is best....but there are exceptions to the rule, always. just be responsible, and KNOW what you're putting into your body.