The Australian conservationist marked his birthday on December 1 by sharing pictures of the Crocodile Hunter - who died in 2006 after being pierced in the chest by the barb of a short-tail stingray - when he turned 21.
Irwin vowed to continue honouring father's legacy as he embarks on the next "chapter" of life.
"Today I turn 21. I was looking through the archives a few days ago and I found these pictures of my dad on his 21st. It can be hard to celebrate the milestones when there's someone missing, but I will carry his memory and passion with me in this new chapter of life. I am one lucky guy to be surrounded by my amazing family today. Time to celebrate!"
View this post on Instagram A post shared by Robert Irwin (@robertirwinphotography)
Opening up to Stellar magazine, Irwin said of his 21st birthday: "It can be heartbreaking. I can't beat around the bush. It's really hard, particularly when it's those milestones."
However, he is thankful to have the support of his mother Terri, 60, and sister Bindi, 26.
"But I have to really praise my entire family, and particularly my mum, on the way in which she's stepped up, to make sure that - while there will always be part of myself missing and I'll never be complete - I never felt like I missed out on anything," he posted.
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