Hollywood’s old guard — emphasis on old — turned out Saturday to celebrate the 70th birthday of David Niven, the son of the Oscar-winning actor of the same name, writes The Post’s Richard Johnson. Younger brother Jamie Niven, the Sotheby’s auctioneer, flew in from New York to join a throng of 150 at the Peninsula Beverly Hills hotel. The birthday boy, who worked as an executive at both Paramount and Columbia, is a producer whose credits include “Rush Hour 3.” Well-wishers included Michael Eisner, Sherry Lansing, Jackie Collins, Betsy Bloomingdale, Jaclyn Smith, Nikki Haskell, Suzanne de Passe and Tina Sinatra.