Aaron Taylor-Johnson, 34, shocks fans as he shares rare photos of his 18-year-old stepdaughter

Aaron Taylor-Johnson celebrated his stepdaughter Jessie's 18th birthday in a very sweet way on Tuesday.

The Marvel star, 34, posted two rare images of himself with Jessie - who he dubbed his 'babygirl' - to Instagram.

In the first photo, a much younger Aaron hugs Jessie as a child.

'Happy birthday babygirl,' he captioned it, followed by two pink heart emojis.

In the next photo, which appears to be more recent, a bearded Aaron hugs her again. He captioned it, '18 now. Still my babygirl.'

Fans in the comments didn't seem to realize that Jessie is Aaron's stepdaughter, and were puzzled as they tried to do the math on how old he would have been when she was conceived.

Others couldn't resist pointing out the 23-year age gap between Aaron and his wife Sam Taylor-Johnson, 57.  

The couple tied the knot in 2012 after meeting three years earlier on the set of the film Nowhere Boy.

They share two children together, while Aaron is also a stepfather to Sam's two daughters.

Sam said in May that she can't believe the 'fascination' people have had with her and Aaron's 23-year age gap.

The couple have a fiercely private relationship and Sam has admitted people's initial interest in the couples relationship was 'intense' because 'people want to understand things when they can't.'

She revealed that the interest has now 'gone away a little bit' as the couple have been together for 14 years.

Discussing her relationship on BBC Radio 4's This Cultural Life, she said: 'I guess that interest was more when we got together, which was now 14 years – at the beginning, was quite intense.

'And that sort of fascination, because he's younger than me, which you know, we couldn't really fathom the fascination. But I guess it's sort of gone away a little bit.

'Mostly when I'm doing press and things like this it sort of resurges a little bit. But yeah, I guess people want to understand things when they can't.

'They want to pick up art when they can't fathom what a certain love story that doesn't fit in a box is.'

Back To Black filmmaker Sam is a mother to Angelica, 26, and Jessie, 18, who she shares with ex-husband Jay Jopling. She shares Wylda, 13, and Romy, 12, with Aaron.