Kourtney Kardashian is not one to waste a #TBT opportunity . On Thursday, the eldest Kardashian sister joined in the Instagram fun with a memorable throwback by sharing a photo of herself from her 20th birthday party. In the 17-year-old snap, she's rocking a hot pink dress and holding onto approximately 900 giant orange and pink balloons. And in proper 1999 fashion, she's giving the camera a kissy face. (We just wish her sisters were in the photo for maximum nostalgic impact!)