"Today marks 18 years of life for me, here in Australia and I really don't know where to begin in thanking so many people for an unforgettable journey so far," Bindi captioned the photo. "So I think that I'll start here, with this photograph."
On Friday, Irwin's family released a heartfelt letter that the Crocodile Hunter wrote to his parents, Bob and Lyn. The note -- which will be published in Bob's upcoming memoir, The Last Crocodile Hunter: A Father and Son Legacy -- was tucked away in a book and was discovered earlier this year.
Written 12 years before his untimely death in 2006, the letter reads, "At 32, I am finally starting to figure it out. In good times and in bad, you were there. Your strength and endurance to raise me will not go unrewarded."