Terri Irwin says her 'heart aches' that late husband Steve isn't here for son Robert's 13th birthday

Terri Irwin couldn't help but think of her late husband, Steve Irwin, while celebrating their son Robert's 13th birthday.

On Wednesday, the 52-year-old conversationalist posted a photo of Steve holding Robert as a baby, and captioned the image with a heartfelt message. "While my heart aches that Steve isn't here for Robert's 13th birthday, each day I'm more aware that Steve lives on thru his incredible son," Terri wrote.

WATCH: Terri Irwin Says She Hasn't Been on a 'Single Date' Since Husband Steve's Death in 2006

Robert's sister, Bindi Irwin, also shared a throwback photo and sweet note on Instagram in celebration of her younger brother's birthday. "Happy Birthday to this amazing human being, my brother," Bindi gushed. "Today you turn 13 and I am so proud of you. This photo was taken many years ago, I still can't believe that today you're a teenager. Since the first day I met you, when you were born, I knew you were destined for great things. You inspire me every day. I love you."

Showing gratitude for his sister and mother, Robert posted a pic of his family standing in front of his 13th birthday cake. "Thank you so much @bindisueirwin and mum for a great birthday!" he wrote on Instagram. "Thank you to everyone for joining us today at Australia Zoo."

Bindi and Robert's parents were married for 14 years when Steve died after being pierced in the chest by a stingray barb in September 2006. While it's been 10 years since the Crocodile Hunter star died, Terri told ET in an October 2015 interview that she has no interest in dating because she's already had her "happily ever after."

WATCH: Bindi Irwin on Mom Terri -- She's the Strongest Human Being I've Ever Known

"I still feel immense love and loyalty to Steve," she explained. "And I've got great kids, I've got a very full life, and while I'm lonely for Steve, I'm not a lonely person."

"I think for other people who have lost someone and have loved again, it's a beautiful thing," Terri added. "But it's just not really on my horizon."

47 PHOTOS
Steve Irwin: The Crocodile Hunter
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Steve Irwin: The Crocodile Hunter
STEVE IRWIN, Australian zookeeper and conservationist, has become a media starÂwith his cable TV show, animation spots, advertisements, and now a feature film, collision course. Photo of Irwin on a promotional junket in Los Angeles, June 28, 2002 appearing on The Tonight Show with Jay Leno and a real live Aligator. (Photo by Ken Hively/Los Angeles Times via Getty Images)
STEVE IRWIN, Australian zookeeper and conservationist, has become a media starÂwith his cable TV show, animation spots, advertisements, and now a feature film, collision course. Photo of Irwin on a promotional junket in Los Angeles, June 28, 2002 appearing on The Tonight Show with Jay Leno and a real live Aligator. (Photo by Ken Hively/Los Angeles Times via Getty Images)
STEVE IRWIN, Australian zookeeper and conservationist, has become a media starÂwith his cable TV show, animation spots, advertisements, and now a feature film, collision course. Photo of Irwin on a promotional junket in Los Angeles, June 28, 2002 appearing on The Tonight Show with Jay Leno and a real live Aligator. (Photo by Ken Hively/Los Angeles Times via Getty Images)
(NO TABLOIDS) Terry Irwin and Steve Irwin talk about their film 'Crocodile Hunter: Collision Course' at 2002 ShoWest in Las Vegas, Nevada. during 2002 ShoWest Press Conference With Steve Irwin at the Paris Hotel in Las Vegas, Nevada. (Photo by Denise
(NO TABLOIDS) Bruce Willis and Steve Irwin, The Crocodile Hunter during 2002 ShoWest Opening Day Luncheon at the Paris Hotel in Las Vegas, Nevada. (Photo by Denise Truscello/WireImage)
(NO TABLOIDS) Bruce Willis and Steve Irwin, the Crocodile Hunter during 2002 ShoWest Opening Day Luncheon at the Paris Hotel in Las Vegas, Nevada. (Photo by Denise Truscello/WireImage)
THE TONIGHT SHOW WITH JAY LENO -- Episode 2061 -- Pictured: (l-r) Wildlife expert Steve Irwin during a segment with host Jay Leno on May 17, 2001 -- (Photo by: Paul Drinkwater/NBC/NBCU Photo Bank via Getty Images)
THE TONIGHT SHOW WITH JAY LENO -- Episode 2061 -- Pictured: (l-r) Host Jay Leno and wildlife expert Steve Irwin with a crocodile on May 17, 2001 -- (Photo by: Paul Drinkwater/NBC/NBCU Photo Bank via Getty Images)
THE TONIGHT SHOW WITH JAY LENO -- Episode 1941 -- Pictured: (l-r) Actor Tobey Maguire, Australian wildlife expert Steve Irwin with host Jay Leno handling a snake on November 14, 2000 -- (Photo by: Paul Drinkwater/NBC/NBCU Photo Bank via Getty Images)
THE TONIGHT SHOW WITH JAY LENO -- Episode 1695-- Pictured: (l-r) Steve Irwin and host Jay Leno feed a large bird on October 6, 1999 (Photo by NBC/NBCU Photo Bank via Getty Images)
THE TONIGHT SHOW WITH JAY LENO -- Episode 1695-- Pictured: (l-r) Wildlife expert Steve Irwin with a tarantula during an interview with host Jay Leno on October 6, 1999 (Photo by NBC/NBCU Photo Bank via Getty Images)
THE TONIGHT SHOW WITH JAY LENO -- Episode 1435 -- Pictured: (l-r) The 'Crocodile Hunter' Steve Irwin during an interview with host Jay Leno on August 14, 1998 (Photo by Margaret Norton/NBC/NBCU Photo Bank via Getty Images)
BEERWAH, AUSTRALIA - SEPTEMBER 05: (FILE PHOTO) A 1996 file photo shows Steve Irwin, known as the Crocodile Hunter, at his Australia Zoo in 1996 in Beerwah, on the Sunshine Coast, Australia. Irwin died September 4, 2006 after being stabbed in the heart by a stingray's barb while diving off Port Douglas in the northern Australia state of Queensland, police said. (Photo by Lisa Maree Williams/Getty Images)
BEERWAH, AUSTRALIA - SEPTEMBER 05: (FILE PHOTO) A 1996 file photo shows Steve Irwin, known as the Crocodile Hunter, at his Australia Zoo in 1996 in Beerwah, on the Sunshine Coast, Australia. Irwin died September 4, 2006 after being stabbed in the heart by a stingray's barb while diving off Port Douglas in the northern Australia state of Queensland, police said. (Photo by Lisa Maree Williams/Getty Images)
SUNSHINE COAST, AUSTRALIA - 1993: (EUROPE AND AUSTRALASIA OUT) 'Crocodile Hunter' Steve Irwin and his wife, Terri Irwin, and 'Jonathon' the Green Iguana at Australia Zoo in Queensland. (Photo by Newspix/Getty Images)
THE TONIGHT SHOW WITH JAY LENO -- Episode 2222 -- Pictured: (l-r) Host Jay Leno and wildlife expert Steve Irwin on March 6, 2002 -- (Photo by: Paul Drinkwater/NBC/NBCU Photo Bank via Getty Images)
Terri Irwin and Steve Irwin, The Crocodile Hunter during 2002 ShoWest Opening Day Luncheon at Paris Hotel in Las Vegas, Nevada, United States. (Photo by Denise Truscello/WireImage)
Terri Irwin and Steve Irwin, The Crocodile Hunter during 2002 ShoWest Opening Day Luncheon at Paris Hotel in Las Vegas, Nevada, United States. (Photo by Denise Truscello/WireImage)
Steve Irwin, The Crocodile Hunter, with Bruce Willis (Photo by Denise Truscello/WireImage)
Steve Irwin during Nickelodeon's 15th Annual Kids Choice Awards - Press Room at Barker Hanger in Santa Monica, California, United States. (Photo by SGranitz/WireImage)
Steve Irwin during Nickelodeon's 15th Annual Kids Choice Awards - Press Room at Barker Hanger in Santa Monica, California, United States. (Photo by SGranitz/WireImage)
Steve Irwin and Will Smith during Nickelodeon's 15th Annual Kids Choice Awards - Backstage at Barker Hangar in Santa Monica, California, United States. (Photo by KMazur/WireImage)
Steve Irwin and Jessica Alba during Nickelodeon's 15th Annual Kids Choice Awards - Backstage at Barker Hangar in Santa Monica, California, United States. (Photo by KMazur/WireImage)
SAN FRANCISCO - JUNE 26: ***EXCLUSIVE*** 'The Crocodile Hunter', Steve Irwin, poses with a three foot long alligator at the San Francisco Zoo on June 26, 2002 in San Francisco, California. Irwin is on a 3-week tour to promote the release of his first feature film, 'The Crocodile Hunter: Collision Course', due in theaters July 12th. (Photo by Justin Sullivan/Getty Images)
SAN FRANCISCO - JUNE 26: ***EXCLUSIVE*** 'The Crocodile Hunter', Steve Irwin, (R) shows a snake to his wife Terri (L) at the San Francisco Zoo on June 26, 2002 in San Francisco, California. Irwin is on a 3-week tour to promote the release of his first feature film, 'The Crocodile Hunter: Collision Course' is due in theaters July 12th. (Photo by Justin Sullivan/Getty Images)
SAN FRANCISCO - JUNE 26: ***EXCLUSIVE*** 'The Crocodile Hunter', Steve Irwin, poses with a three foot long alligator at the San Francisco Zoo on June 26, 2002 in San Francisco, California. Irwin is on a 3-week tour to promote the release of his first feature film, 'The Crocodile Hunter: Collision Course' is due in theaters July 12th. (Photo by Justin Sullivan/Getty Images)
Steve Irwin, Terri Irwin, and daughter at the Arclight Cinerama Dome in Hollywood, California (Photo by Ron Galella/WireImage)
Steve Irwin, Terri Irwin & daughter during 'The Crocodile Hunter: Collision Course' Premiere at Arclight Cinerama Dome in Hollywood, California, United States. (Photo by SGranitz/WireImage)
Steve Irwin, Terri Irwin & daughter Bindi Irwin during 'The Crocodile Hunter: Collision Course' Premiere at Arclight Cinerama Dome in Hollywood, California, United States. (Photo by SGranitz/WireImage)
Steve Irwin with wife Terri Irwin and daughter Bindi Irwin (Photo by Justin Kahn/WireImage)
BURBANK, CA - FEBRUARY 4: TV's 'Crocodile Hunter' Steve Irwin (L) wraps an anaconda snake around host Jay Leno on 'The Tonight Show with Jay Leno' at the NBC studios February 4, 2003 in Burbank, California. (Photo by Kevin Winter/Getty Images)
Steve Irwin with his wife Terri and daughter Bindi (Photo by John Stanton/WireImage)
Steve Irwin with his wife Terri and daughter Bindi (Photo by John Stanton/WireImage)
Steve Irwin with his wife Terri and daughter Bindi (Photo by John Stanton/WireImage)
(AUSTRALIA & NEW ZEALAND OUT) Steve and Terri Irwin pose for a photograph at the Botanic Gardens restaurant in Sydney, while promoting their new movie 'The Crocodile Hunter', 9 September 2002. SMH Picture by SEAN DAVEY (Photo by Fairfax Media/Fairfax Media via Getty Images)
SUNSHINE COAST, AUSTRALIA - JANUARY 3: (EUROPE AND AUSTRALASIA OUT) Steve Irwin with wife Terri, daughter Bindi & father gives a press conference outside his Australia Zoo at Beerwah on the Sunshine Coast defending his actions in taking son Bob (1 mth) into fenced crocodile enclosure on January 2, 2004. (Photo by Graeme Parkes/Newspix/Getty Images)
SUNSHINE COAST, AUSTRALIA - JANUARY 2: (EUROPE AND AUSTRALASIA OUT) Steve Irwin and wife Terri, with daughter Bindi and baby son Bob, at a blessing ceremony by Tibetan Buddhist nuns, for the Irwin youngster. (Photo by Graeme Parkes/Newspix/Getty Images)
AUSTRALIA - NOVEMBER 20: (EUROPE AND AUSTRALASIA OUT) Steve Irwin, who is Queensland's Australian of the Year 2004 state finalist, is congratulated by his daughter Bindi Irwin and wife Terri Irwin. (Photo by Nathan Richter/Newspix/Getty Images)
SUNSHINE COAST, AUSTRALIA - JANUARY 3: (EUROPE AND AUSTRALASIA OUT) Steve Irwin and daughter Bindi gives a press conference outside his Australia Zoo at Beerwah on the Sunshine Coast defending his actions in taking son Bob (1 mth) into fenced crocodile enclosure on January 2, 2004. (Photo by Graeme Parkes/Newspix/Getty Images)
Steve Irwin during The 78th Annual Macy's Thanksgiving Day Parade at Manhattan in New York City, New York, United States. (Photo by David Pomponio/FilmMagic)
THE TONIGHT SHOW WITH JAY LENO -- Episode 2879 -- Pictured: (l-r) Wildlife expert Steve Irwin during an interview with host Jay Leno on February 18, 2005 -- (Photo by: Paul Drinkwater/NBC/NBCU Photo Bank via Getty Images)
SUNSHINE COAST, AUSTRALIA - DECEMBER 1, 2005: (EUROPE AND AUSTRALASIA OUT) 'Crocodile Hunter' Steve Irwin, with his wife Terri Irwin, daughter Bindi Irwin, and son Bob Irwin, on Bob's 2nd birthday, at Australia Zoo sign at Beerwah on the Sunshine Coast. (Photo by Newspix/Getty Images)
Steve Irwin during G'Day LA: Australia Week 2006 - Penfolds Icon Gala Dinner - Arrivals in Los Angelees, California, United States. (Photo by SGranitz/WireImage)
©2006 RAMEY PHOTO Australian actor Hugh Jackman (L) and Steve Irwin, the crocodile hunter, give thumbs up as they arrive for the Penfolds Icon Gala Dinner at the Hollywood Palladium in Los Angeles, California January 14, 2006. The event brings together and honors Australians who have made significant international contributions across various industries. Jackman received the Excellence in Entertainment award. 01/14/06 JR (Photo by ©2006 Phil Ramey/RameyPix/Corbis via Getty Images)
BEERWAH, AUSTRALIA - JUNE 19: Steve Irwin poses with his family at Australia Zoo June 19, 2006 in Beerwah, Australia. (Photo by Australia Zoo via Getty Images)
BEERWAH, AUSTRALIA - AUGUST 02: Steve Irwin poses with his son Bob at Australia Zoo August 2, 2006 in Beerwah, Australia. (Photo by Australia Zoo via Getty Images)
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