Steve Irwin’s son Robert has admitted it’s “hard to celebrate” milestone birthdays while “missing” his late father on his 21st birthday.

The Australian conservationist marked his birthday on Dec. 1 by sharing throwback pictures of the famous Crocodile Hunter, who tragically died aged 44 in 2006 after being pierced in the chest by the barb of a short-tail stingray, when he turned 21 and vowed to continue honoring his legacy as he embarks on the next “chapter” of his life.

Robert penned on Instagram: “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!”

The zookeeper, who was just 2 when his dad died, heartbreakingly admitted it’s “difficult navigating” life’s big moments without a “father figure” and that he will “never be complete.”

Opening up to Stellar magazine, Robert said of his special 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 mom Terri, 60, and older sister Bindi, 26.

Robert continued: “But I have to really praise my entire family, and particularly my mom, 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.”

Robert split from Rorie Buckey, the niece of late actor Heath Ledger, earlier this year after more than a year of dating and admitted he’s looking forward to settling down with the “perfect person” one day and branded his parents and Bindi and her spouse Chandler Powell “couple goals.”

He told the publication: “My mom and dad were just this incredible power couple. And then I look at Bindi and Chandler, and it’s like they’re literally the same person. They’re made for each other. I mean, if that isn’t your couple’s goals, what would be?

“I really want to find the perfect person, but I’m not in any rush for it. I’m very open to it. The thing that I’ve come to discover in that space is you’ve just got to let it happen.”