If it were really only about the candy then Joshua Gans nails it:
I have to say that, as an economist, this entire evening is a curious one. Let us be clear what happened. We all bought bags of candy. We then gave it out while sending our own children to collect more. A simple adding up constraint suggests that the total consequence of the evening was a massive redistribution of individual candy but probably not much change in the aggregate household acquisition. Basically, we just bought our kids a ridiculous amount of special treats and tacked on a hunting task to the endeavour. The candy industry must be thrilled with this turn of events.
Sadly, there was little innovation to be seen. One house gave out little toys which thrilled everyone. Another gave out fair trade chocolate. But because of the completely irrational fears about poison or something (and let me tell you, from an economic point of view, Halloween is the worst possible time to engage in murder without consequence), no one could make anything and it was difficult to step apart from the crowd. This only occurred to me too late. What remains is a flow of candy in a game of musical chairs where everyone can sit. I can see why 7th graders might get tired of it after a decade or so.
For my kids, the night, indeed the weeks preceding the event, seemed far more about the costume than the candy, not that they weren’t thrilled about the candy. Probably there’s a natural progression. Before some age (beats me what it is–will tell you when I get there), the costume dominates. Then it’s about the candy for a few years. Then it all seems like too much work for what otherwise amounts to $5 worth of sugar. Gans says that’s about 7th grade. OK! Good to know. I’ll look for it.