Mary-Kate and Ashley Olsen celebrated their 21st birthday with an intimate dinner party on Wednesday night. The twins decided to keep mark their milestone age by sharing a quiet meal with family and friends at Hollywood's Chateau Marmont, where guests included their younger sister Lizzie, 19, and 23-year-old brother Trent.Mary-Kate and Ashley - who shot to fame aged just nine-months playing Michelle in hit US TV show 'Full House' - were the center of attention as gifts and cards were lavished upon them all night. At 11pm, two red velvet cakes were brought out - one covered in white icing, the other in black - and the whole table sang 'Happy Birthday' to the girls, who then blew out their 21 candles.During the meal, each guest took it in turns to raise a glass of champagne to the birthday girls and the sisters happily posed for pictures, which were taken on their vintage Polaroid camera. An onlooker said: "It looked like a family dinner. You could tell they were all really close. The girls couldn't stop smiling."Mary-Kate looked on lovingly as the mystery man sitting next to her gave the last speech, before kissing her. Before their party, the girls had told their fans they planned to make the most of their special day.They wrote on their website: "Today, for the first time in a looooong time, we're taking a day off - doing absolutely nothing and looking forward to every second of it!"