Marc Jacobs has several reasons to celebrate today. In addition to his recent engagement, he'll also be blowing out 46 candles on his birthday cake today. Others marking the big day: NBC chief Jeff Zucker is 44. Hugh Hefner turns 83. Cynthia Nixon is 43. Leighton Meester is turning 23. Albert Hammond Jr. is turning 29. Joe Scarborough is 46. Porn queen Jenna Jameson is 35. Dennis Quaid turns 55. Fashion designer Martin Margiela is 52. Restaurateur Taavo Somer turns 36. Twilight actress Kristen Stewart is 19. Jesse McCartney turns 22. America's Next Top Model winner Yoanna House is 29. And former model-turned-ANTM judge Paulina Porizkova is 44.