She's one of the most well-known faces in Hollywood but Jennifer Garner left her Instagram followers scratching their heads after sharing a throwback photo on social media.Garner posted a photo of her aged 21 alongside an image of Adele's iconic album cover for 21, taken at the same age.

"You've got it or you don't," the Alias star captioned the photo along with the hashtags "#doesnthaveit", "#butwhatareyougonnado" and "#hiadele".

Garner's post soon attracted plenty of comments jokingly commiserating her for not looking as glamorous as Adele at 21, reports News.com.au.

But followers soon noticed something else about the photo — the actress' incredibly youthful appearance.

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"That's why you still look like you're in your twenties!"

"OMG you looked like you were 13 at 21!"

"I'm getting major 13 going on 30 vibes! Love that movie."

"OK but u look like you are 12 at 21!" actress Juliette Lewis wrote.

"Well your 21 looks 15 so …" another commented.

Now 47, Garner credits her appearance these days to following a golden rule of skincare.

"First off, I wash my face every single night, no matter what," she told Marie Clarie earlier this year.

"I honestly wear no makeup unless there's a reason to, but if I happen to have eye makeup on that day I'll remove all of it."

Garner also swears by using a skincare holy grail ingredient — retinol.

Jennifer Garner in 2002, (left), and the actress last year, (right). Photos / Getty Images
Jennifer Garner in 2002, (left), and the actress last year, (right). Photos / Getty Images

"Being in my mid-40s, retinol really is the most important product for anti-ageing," she said.

Garner's breakout Hollywood role came in 2001 when she landed the role of Sydney Bristow in TV series Alias.

Since then she's starred in a string of successful movies which have included Juno, 13 Going on 30 and Dallas Buyers Club.

Garner began dating Ben Affleck in 2004 and married in 2005 before announcing their separation a decade later.

The former Hollywood power couple share three children together; Violet, Seraphina and Samuel.