The catalyst is yet another article telling adults who enjoy YA literature that they should be ashamed of themselves. This is a particularly lightweight instance of the genre, but I was struck (as I often have been by similar statements in the past) by the writer's confession that "I remember, when I was a young adult, being desperate to earn my way into the adult stacks." (The word "earn" is significant in itself, implying as it does that getting older is some kind of meritorious deed rather than the inevitable consequence of failing to die.)
I do remember thinking as a child that grown-ups had all kinds of enviable privileges (going to bed when they wanted, and the like), but don't ever remember wanting to be grown up for its own sake. However, I've heard many people over the years say that they couldn't wait to do so - and my impression (possibly wrong) was that they saw grown-upness as a more desirable existential state, rather than (or as well as) the chronological key to a set of legal rights and privileges.
The pet theory (really a hypothesis) that's been doggy-paddling through my thoughts is that there's likely to be a strong correlation between people's attraction to grown-upness and the taking up of smoking during adolescence. After all, what attractions could smoking have to a non-smoker? Anyone of my generation or younger will have known from childhood that it's expensive, addictive, smelly, unhealthy and all too frequently fatal in the long term. The main reason I can see for wanting to take it up at 15 or so is that you imagine it makes you look sophisticated and cool in a specifically grown-up way. There are other possibilities - it's no doubt an act of rebellion, for some, for example - but I suspect that that looking grown up is a big driver.
But how to test the hypothesis? A random appeal to the experience of my LJ friends may not be the most scientific approach, but then it's not the most scientific theory in the first place...