Minors Consenting to the use of substances.

Since we very well argue that a minor can consent to sex what about the thoughts that a minor could consent to the use of substances. (I.E Alcohol, Cannabis, tobacco, ect.) I'm curious as far as your thoughts on this (I know comparing alcohol, cannabis, and tobacco is like comparing three competly different things from deep space but they are the most commonly used substances in the western world.) I'm curious on your opinions. I also know that other countries have various drinking/smoking ages aswell. Do you think these should stay or be abolished? My opinion the jury is still out, mainly because I couldnt write a law juding myself because I've always been responsible with the uses of such substances, and when writing changing such laws you always have to take account for the reckless and irresponsible...