well, I read this thread a couple of days ago but didn't have time to write a reply, so I have come back.
My first thoughts when reading it were, gotta be in your 20s atleast but then I started to think about my friends and other people I know and how they all handle their drug use.
my own experience:
before I turned 18 I was still pretty into my sport and cos of that I didn't go to that many of the pissup parties that were going on. After I turned 18 and had virtually quit training and competition, I, along with my friends took up drinking with a vengeance. We were damn proud of the fact that we could drink most of the guys we knew under the table, thought nothing of drinking til we passed out, and could easily have got into trouble many a time.
a couple of my closest friends starting experimenting with pills and speed when they were 19 and (after many months of curiousity) I had my first e at 20. On about my 3rd one I had a bad experience and didn't really touch anything again until june this year. About a month after that I found bluelight and since then I have been reading as much as I can find on harm minimisation etc etc.
so for me, physically, for sure I coulda handled pills at 17/18, but mentally there is no way I would have been ready for it. I know the way we used to just go out and completely F*** ourselves up and I don't doubt we would have been the same with pills.
that said, while I am on here all the time educating myself, some of my friends just dont care. I have tried to tell some of them things i have learned, not being pushy, just adding it into the conversation when I feel it is appropriate but they aren't at all interested. I find it frustrating, but it is like the saying "you can lead a horse to water, but you cant make it drink" i can be there with info to help, but I can't make them listen.
so what it comes down to is definitely the mental maturity level. I think once you have reached your physical maturity, if you've got your head together, know how to look after yourself, understand the resposibility of what you are doing, then go for your life.
thats my longest rant in a while!!
stay safe everyone, look out for your health, remember the value of your family, and look after your mates cos losing any of the above is pretty damn painful.