Paris Hilton is throwing a star-studded Las Vegas birthday party for her boyfriend STAVROS NIARCHOS when the Greek shipping heir turns 21 later this month (APR06). Ironically, the couple created a stir recently when they were denied access to Sin City's celebrity hotspots because Niarchos was underage, but that isn't stopping them from partying on Vegas' famous Strip. Hilton explains, "(His birthday) is going to be amazing. I've already planned a big weekend in Vegas. He's so excited he can finally gamble and not get in trouble." The couple recently returned from a romantic holiday in the Bahamas, where the legal drinking and gambling age is 18 years old. Hilton adds, "(Legally) We were fine. It was fun!"