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It was a bittersweet day for Terri Irwin.

On Wednesday, the widow of the late Steve Irwin celebrated their youngest son’s 13th birthday and it was clear someone was missing.

“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,” she tweeted alongside a photo of her late husband holding Robert as an infant.

The Irwin’s eldest daughter, Bindi, also shared a sweet message to her brother on Instagram.

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“Happy Birthday to this amazing human being, my brother. 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. Ps. You now have Instagram!” she wrote.

Steve Irwin died in 2006 after he was stung by a stingray off the coast of Australia. According to PEOPLE, Terri Irwin has not been out on a date since he passed.

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