Chicago ? Step into Frank’s bar, and you’ll get a glimpse of what’s happening to Halloween.

The popular tavern in Chicago’s Lincoln Park neighborhood has been decked out for weeks in black and orange, and patrons can order drinks with such names as “Sex on a Tombstone.” On the night itself, manager Robby Ehlert expects to see a number of costumes that won’t be G-rated.

“A lot of the costumes are, uhhh, not costumes kids would wear,” he said. “You’ll see sexy cops, sexy pirates — anything sexy basically.”

Increasingly, Halloween is a holiday for adults, sometimes celebrated with children but often without them.

“I’ve never seen a season like this,” said Joe Marver, founder of Spirit Halloween Superstores, a chain of nearly 200 specialty stores nationwide that open just for the weeks preceding the holiday.

Marver said most adults used to wait until the last minute to throw together a costume. But this year, he’s already had to reorder some lines of adult-sized garb. (He, too, says pirates are big this year, apparently due to the movie “Pirates of the Caribbean: The Curse of the Black Pearl.”)

Adults now represent up to 65 percent of his costume sales, a noticeable shift compared with years past.

“This is an adult holiday,” Marver said. “It’s party time.”

A first-time survey done in recent weeks by the National Retail Federation found that young adults were fueling the trend.

Of those surveyed, 57 percent in the 18-to-24 category said they planned to dress in costume, and nearly half said they’d be attending a Halloween party. In the 25- to 34-year-old category, it was 45 percent and 37 percent, respectively.

Survey respondents in their 40s and 50s were more likely to decorate their homes and yards.

“I think our generation is a little more hesitant to let go of childhood than past generations,” said Ehlert, the Chicago bar manager, who’s 30. “We want to hang onto Peter Pan as long we can.”