The Greek shipping heir, 19, attempted to enter nightspot Pure for the bash, which Hilton's younger sister Nicky was hosting, but was turned away because he was two years shy of the legal drinking age.
According to the Las Vegas Review Journal, the Hilton sisters loudly stormed out of Pure at 3.30am on New Year's Day because security bosses turned Niarchos away.
But Las Vegas police have launched an investigation after Cox Cable Channel 19's In Business Las Vegas reported the pair had sneaked into the club illegally later, before entering another popular nightspot, Hard Rock Hotel's Body English.
Hard Rock President Kevin Kelley said his club was cooperating with authorities, as well as conducting an internal investigation.
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