In the image, Aamir is seen in his Mangal Pandey avatar. He is sporting long hair and handlebar moustache while Ira is seen sitting next to her father in a salwar kameez.
"Happy 21st Ira! Can't believe you got there so fast! You will always remain six for me! Love you. Papa," Aamir wrote.
Ira is the daughter of Aamir and his first wife Reena Dutta. The two also share a son named Junaid. They got divorced in 2002. Aamir got married to Kiran Rao in 2005.
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On the work front, the 54-year-old actor will next be seen in "Lal Singh Chaddha", a remake of the 1994 Hollywood blockbuster "Forrest Gump". The film will be penned by Atul Kulkarni and directed by Advait Chandan of "Secret Superstar" fame.
Aamir's "Dangal" co-star Fatima Sana Shaikh also wished Ira through her Instagram account.
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"Happy birthday cutie," she wrote, and shared two images of herself hugging Ira.
Fatima Sana Shaikh wishes Ira Khan on her birthday
Fatima Sana Shaikh wishes Ira Khan on her birthday