Steve Irwin's son Robert has admitted it's "hard to celebrate" milestone birthdays while "missing" his late father on his 21st birthday

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 two 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 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."

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 mum 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."