Knightley played Julie in the hit 2003 rom-com, who weds Peter (Chiwetel Ejiofor) only to find his best friend (Andrew Lincoln) is madly in love with her.
Brodie-Sangster played a school child who learns the drums in order to impress a fellow pupil he has fallen for.
But despite their evident age difference in the Richard Curtis film – there is only a five year difference between the actors in real life.
Knightley was born in 1985 making her 18 when she took on the role, while Brodie-Sangster was 13, having been born in 1990.
The mind blowing fact was pointed out by a Twitter user who posted a picture of the pair as their characters alongside the caption: “HOW THE F***.”
The post was retweeted over 31,000 times and liked by more than 70,000 users.
Fans of the film voiced their shock with many admitting they thought Brodie-Sangster was a child in the film and Knightley way past her teenage years.
One Twitter user posted: “I’m struggling to believe this how can this be true what on earth did they do to him.”
Another wrote: “I’m honestly so shocked fully thought he was defo 6 in it.”
A third joked: “Yeah I think we can agree Thomas is an actual Vampire, me and my family have had multiple conversations on this same topic.”
The cast came together for a reunion spin-off in aid of Comic Relief last year with Knightley and Brodie-Sangster joined by fellow co-stars including Ejiofor, Lincoln, Hugh Grant, Martine McCutcheon and Liam Neeson.