Consent's not really the issue

Even an ant can consent to sex or consuming a substance (mouse traps work well because mice consent to eating some cheese). Consent is really not the most important thing here (though it's a good requirement) but moreso that someone's informed about what's happening to them so they can deal with circumstances, make a good initial decision, change their minds along the way, predict repercussions, etc.

I don't think it's right to bar acts or substances merely on the basis of age, we should have other criteria for deciding these things. I can accept a lesser evil of using ages until a better system is put in place, but only as a temporary solution (much like you can use sexist or racist laws as temporary solutions when you lack resources). The problem is that it's not really being used as a temporary means of change, but rather that it's been sticking around and stuff.

Alcohol and tobacco are probably the conversation points seeing as how cannabis is illegal.