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."