Keira Knightley fans disturbed after realising how old she was when filming Pirates of the Caribbean

Fans of Keira Knightley are positively stunned upon learning the age of the actress during her first time on the Pirates of the Caribbean set.

The fantasy drama, starring Knightley as Elizabeth Swann, the daughter of Governor Weatherby Swann, also featured Orlando Bloom as Will Turner and Johnny Depp as Captain Jack Sparrow.

With the first film, Pirates of the Caribbean: The Curse of the Black Pearl having premiered in 2003, people are really shocked to have learned just how old Knightley was while filming the role which led the actress to therapy.

Despite the final Pirates of the Caribbean film being released in 2017, it still holds an extremely passionate fanbase who often discuss the film and trivia online.

On Reddit, one user drew attention to Knightley's age on set during filming in 2003.

They wrote: "During the recording of Pirates of the Caribbean: The Curse of the Black Pearl, Kiera Knightley who played Elizabeth Swann was only 17 which I find quite surprising ... Also her mother traveled with her to every shooting location, from LA to Saint Vincent and the Grenadines."

By the time the film was released, Knightley was 18-years-old.

Another person chimed in: "I think a lot of people don't realise how *young Keira Knightley was when she broke out. She was also in Love Actually around the same time, that came out in 2003 so she was 18 or possibly 17 when it was shot. It's arguably weirder there (at least from today's perspective) because she plays the bride that the best man is in love with..."

The 39 year old actress admitting to not being an extrovert (Gareth Cattermole/Getty Images)

Another person said: "She said in a BBC Radio One interview that she was also 17 for Love Actually, which I agree makes it a little weird that 28 year old Andrew Lincoln and 25 year old Chiwetel Ejiofor were the love interests. She was also 16 in Bend it Like Beckham."

Today, Knightley is 39-years-old, and one of the most famous actresses on the planet.

In a recent interview with Variety, she opened up about her role as Elizabeth Swann.

She said: "I'm not an extrovert, so I found that level of scrutiny and that level of fame really hard.

"It was an age where you are becoming, you haven’t become, and you need to make mistakes. It’s a very precarious age, particularly for women."

She then went on to add: "Looking back, would I do anything different? No, I wouldn't because I'm unbelievably lucky now, and my career is in a place where I really enjoy it, and I have a level of fame that's much less intense.

"I can deal with it now, and that's great. But at the time, it was not so great, and took many years of therapy to figure it out."