Placebo of Adulthood

The last 18 months of my life have been so unbelievably dramatic, and it all seems to stem from this idea that we've finished school, we've turned 18, and magically, we're mature adults. I don't get it. At all. And it's really making me nostalgic for the days of being a kid. When there wasn't some kind of invisible barrier between the sexes (that society has since magically placed there), and when life just seemed a lot simpler. But it more makes me wonder when and why this placebo of 'adulthood' settled on us all, and how on earth we let it happen without a fight.