This bond is unbreakable! Robert Irwin commemorated Bindi Irwin's 22nd birthday on July 24 by sharing a sweet tribute to his older sister, as well as a sentimental throwback photo with their late father, Steve Irwin. "Happy birthday @bindisueirwin. Thank you for being my best friend from the moment I entered the world. I'll always treasure the story of the first time you met me — it was around the time this photo was taken, just after I was born and you decided that it was your job to name me... and then promptly named me Brian!"