The youngest Irwin sibling shared an old photo of himself and his mum, with many fans commenting that Terri has barely aged in the years since.
In the photo, Robert is a toddler being held by Terri, with the pair wearing winter clothes, including beanies.
He also shared a recent snap of the pair, with Terri wearing the same beanie from the first photo.
"Happy birthday mum! You’ll always be the biggest inspiration and guiding light for me and Bindi. You are incredible, love you!" he captioned the photos.
"She doesn't age," one user said, with another agreeing, writing, "Momma Irwin hasn't aged a bit! I hope she had the most wonderful birthday!"
"She has the secret to eternal youth!" a third added.
"Teri, you don’t look a day older," someone else wrote. "Happy birthday and here’s to more of ‘em."
"She is an inspiration, just like your Dad," another said. "She never ages!!! Happy birthday Terri."
"Momma Irwin hasn’t aged a bit!" yet another follower wrote. "I hope she had the most wonderful birthday!
Bindi's post featured a photo of the two taken recently while holding a reptile.
"Mum, HAPPY BIRTHDAY!" Bindi began her lengthy post. "You are the most extraordinary mama and now watching you become the very best grandmother, my heart is overflowing. With each new year I love, admire, and appreciate more and more the remarkable woman you are."
She continued, "Thank you for your incredible strength and guidance, thoughtful patience and encouragement, and above all - your unconditional love. You do so much for our family, and for everyone around you. It’s amazing watching you effortlessly, selflessly care for others, wildlife, humans and our planet alike.
"I love celebrating YOU today and every day. Can’t wait to make this next year truly unforgettable, filled with family, and revelling in every joyful moment we share. I love you, every minute, for all of my days."
Many of Bindi's followers also wished Terri a happy birthday, with one user writing, "Happy birthday Terri! What an inspiring woman you are. Genuine, kind human, the world is lucky to have you fighting for it."
"Through the very best of times and the very worst of times this amazing lady carried on," another said. "She is a rare gem, and to be greatly admired! Happy Birthday!"
"Happy birthday to one of the best women to be on this earth," a third added. "The strength she's shown all these years is inspiring."
Never miss a thing. Sign up to Yahoo Lifestyle’s daily newsletter.
Or if you have a story idea, email us at firstname.lastname@example.org.