Disney sensation Miley Cyrus is now officially the wealthiest child in the world - she is reportedly set to be a billionaire by the year's end. Raking in $18 million in 2007 alone, the 15-year-old singer/actress has climbed her way to the top of a list of Hollywood's richest kids, with her Hannah Montana franchise estimated to be worth $1 billion by 2009. A spokesperson for Disney says, "Kids are earning more and more and can command huge fees in Hollywood. But no one could have predicted how popular Miley would be. She's a true star." Cyrus' mother Leticia adds, "A lot goes into her investment fund, which she can't touch until she's 18." The teen hit the headlines earlier this week when it was announced she had signed a seven-figure deal with the Disney Book Group to write her autobiography, slated for a spring 2009 release.