Once upon a time, it seemed as though Zac Efron was the ultimate fantasy boyfriend of every straight teenage girl growing up in the mid-2000s. After Efron portrayed Troy Bolton in the hit Disney tween movie High School Musical, he became the idol of millions overnight thanks to his boyish good looks, charming smile and golden-throated singing voice.

Seriously, even I have to admit that he's pretty damn fanciable. Eventually, every girl's bedroom wall in the land was adorned with pin-up posters of Efron. They were totally ubiquitous for a while ... until newer, younger heartthrobs gradually replaced him that is.

Nowadays Efron is known for much more than just HSM. However, it seems as though not everyone threw out those old pin-up posters, as this heartwarming Instagram post proves. This week, Efron took to his own official Instagram account, to share an adorable video of his grandfather's birthday.

In the footage, the senior Zefron can be seen celebrating his 91st birthday by blowing out the candles on his birthday cake. However, eagle-eyed fans noticed that, hanging on the wall behind him, was one of the pin-up posters of his famous grandson Zac. It seems as though his grandad kept it so he could always remember him. That's so sweet! I think I've got something in my eye ...

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Efron captioned the super-cute Instagram post: "Happy 91st Bday to the Original Efron! The man who made it all possible! After the army-he Married my beautiful Grandma Dotty, raised three young Efrons- one of whom is one heck of a guy- my dad. And from the day I came into this world, taught me everything. [sic]"

He added: "How to tell stories, ski, laugh, love, work hard, and appreciate the beautiful things in life: paying it forward, treat others as one would like others to treat oneself, and live every day to the fullest. I’ve never seen anyone light up a room like you Grandpa. Thanks for being my idol, mentor, best friend, and the BEST SKIER in the family. Happy 91 years Grandpa! I love you. I’ll See you and Grandma soon! Can’t wait to follow your tracks down the ? and beyond. [sic]"

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On social media, Efron's many fans experienced cuteness overload at the sight of the posters. One Twitter-user wrote: "I don’t know what’s cuter: @ZacEfron ‘s message to his grandpa or the fact that his grandpa has old posters of Zac from the hsm era hanging on the wall [sic]," while another added: "Zac Efron's grandpa has a signed poster of him in his HSM era hanging in his kitchen and as it turns out, I am also Zac Efron's grandpa."

Huh, you know I think my sister actually used to own this exact poster too. I thought it looked familiar. Maybe Efron senior stole it from her? I haven't seen it in a while ...

Zac Efron's grandpa has his teen pics on his wall and fans can relate