MACAULAY CULKIN has regressed to childlike tendencies in his adult year, sources say.

Hot on the heels of The ENQUIRER’s bombshell world exclusive linking the former child star to heroin use comes grim and disturbing news from a pal.

An insider said Mac has been "trying to relive [his youthful] days" hosting dress-up parties in which guests are invited to come dressed as a movie character or a dinosaur.

One former show business colleague called Mac "a sad and lonely figure.’

"He likes to live as a child," the source said. "Growing up has NOT been good to Macaulay."

Contributing to his sadness, insiders say, is his inability to get acting gigs.

The 32-year-old, who showed off his dramatic chops in movies like 1992's The Good Son and 2007's Sex and Breakfast but had his last major role three years ago on NBC's floperoo Kings, reported.

The ENQIRER previously reported  that his bustup to hottie thesp Mila Kunis triggered his skin down the slippery slopes.

Culkin did make some bucks off Home Alone which made more than $476 million worldwide, and spawned two sequels, cementing Macaulay as the premiere child star of the 90s.

No, not so much…