When I dressed up to hand out candy in high school, mid-late-1990s, kids had basically stopped going out like that. Only a few would show up, none in big groups of friends -- just accompanied (all the way to the door) by their parents -- and they started very early, around 4 or 5pm, and were done before dark. And these were only the kids within a roughly 3 or 4-block radius. None of this has changed since.
Because our culture has become steadily more geared to adults, rather than children or adolescents, it was only a matter of time before adults decided to take Halloween for themselves, as a sort of Carnival. This was underfoot at least as early as 1994, when the Halloween episode of My So-Called Life featured the two Baby Boomer parents renting pirate and sexy princess costumes, which helps them re-invigorate their flaccid sex life. The streets are still filled with little kids, though, so this must have been a transition year.
To put some numbers on how much things have changed, I did a Google search for "halloween costumes" and went through the first page of results. Five sites had numbers available. Here is a table of the number of costumes available for boys, girls, men, and women, followed by a list of the adult : young ratio and the female : male ratio.
Adult : Young
Female : Male
No matter which high-traffic site we look at, adult women are by far the target audience for Halloween costumes. Regardless of age, females are buying more than males -- "look at meee!" -- but age is a much stronger predictor, and not in the direction you'd think, i.e., exhibitionistic youth. You might argue that it's just a return to the masked ball tradition, but those men didn't look like queers, nor the women like skanks. No, this is just old people annoyingly and embarrassingly clinging to the spotlight for attention when they should bow out and go where they'll do well (politics, running a business, etc.).
It's yet another example of how young people today are in many ways more mature than the irresponsible adults. For instance, STDs among young people have plummeted in recent years, while among the middle-aged they have shot up. Of course, all we hear from the morons with megaphones is how kids these days don't want to grow up, and if only they behaved as responsibly as we do. Given how few privileges and how little opportunities for fun kids have today, and given how many entertainment and leisure outlets are geared toward adults, there has never been any time in history when young people have wanted so badly to become adults -- so they can have something to do.
I'd melt my eyeballs before I took a long look at the typical over-25 woman in a barely-there costume. Almost every woman that age is too old to have much worth showing off, especially in the legs (which is why they abandon short-shorts around 25). By then their skin's succumbed to too much sun exposure, and their collagen has been zapped to hell by all the glycation that their high-carb diet has brought about. And sorry, but no one with an increasingly masculine face can pretend to be Snow White.
But at least the high schoolers still get dressed up for Halloween. If only you could freeze them at 17, like this one from the teen dance club (who I mentioned here). Now that'll put lead in yer pencil.