There's some interesting data in this USA Today article about a topic I've covered for awhile now (just search the blog for Halloween). The clearest sign of the holiday's "adultification" is that at Halloweenexpress.com, over 60% of costumes sold now are for adults, vs. under 30% a decade ago. If they had sales data two decades ago, they would've found more like 3% of costumes going to full-grown adults.
It would be a mistake, though, to view the major change in the nature of Halloween as adults simply taking over the holiday. I don't think I've emphasized it before, but it's not as though the roles that the participants play have stayed the same, only now the participants are grown-ups instead of children. It's that the roles themselves have fundamentally changed.
When Halloween was a real holiday, social interaction centered around trick-or-treating, where the children played the role of guest and the candy-passer-outters the role of hosts. Guest-host relationships are fragile and so are always the first to go during a breakdown of trust. The guests begin to believe that their hosts would take advantage of them, abusing their higher status. And for their part, the hosts begin to believe that their guests would take advantage of the hosts' vulnerable state after letting strangers get so close to their personal space.
Aside from trick-or-treating, hitch-hiking is another guest-host relationship that has completely evaporated over the past 20 years. It used to be common during the early 20th C, but disappeared during the cocooning mid-century, only to be re-discovered in the 1960s.
At any rate, today's grown-up Halloween participants do not enter into any sort of special guest-host relationship, other than at most your garden variety house party. Even there, the role of host is minimized -- they're just supposed to allow their home to be used for the small get-together, not to hand out gifts, etc. And the guests are supposed to minimize their role as well, just show up and mingle, rather than make a display of "please" and "thank you".
More importantly, the guests are not strangers, but close friends of the host. Some or most of the trick-or-treaters would have been known to the hosts, but they generally did not have frequent social contact with each other. They therefore had to overcome more potential distrust than close friends would have to, making the old Halloween ritual a stronger and purer example of guest-host interactions.
In such relationships, the two parties allow themselves to be somewhat vulnerable, trusting that the other party will not exploit them once their guard is down, and feeling something of a renewed faith in others when it's over and neither has in fact taken advantage of the other.
Unlike those relationships, today's Halloween ritual is more of an antagonistic status contest. Like, who can think up and pull off the most uniquely ironic costume? Or who can make the most obscure allusion? This is the opposite of the children's pattern, where they wear costumes that aren't so different from each other's. It pushes everyone in the direction of the catty red carpet diva who, despite trying to quiet her self-consciousness, remains anxious about what all those other bitches are wearing, and whether the audience will judge her look to be among the best.
There's a small degree of that competitiveness in any party atmosphere, but at your ordinary party, the range of what you're expected to wear is a lot narrower. It's generally considered in bad taste to show up to a normal party dressed incredibly different from anyone else, trying to stand out that much.
On Halloween, though, anything goes, and that invites people to go as far-out as they please in order to distinguish themselves from their competitors. This also means you have to plan a lot more, instead of thoughtlessly opting for one of a limited range of choices, which would have allowed you to forget yourself and get lost in the moment. Now you've got the spotlight of self-consciousness trained on your face the entire night.
One final note -- a holiday of reversals also tells us about what ordinary life is like, through contrast. In the good old days, people in everyday life were a lot freer to wear whatever they wanted, keep their hair long or cut it short, and listen to this, that, or the other music group. And in the current age, people are a lot more conformist in how they dress and groom themselves, how they entertain themselves, and so on.