The pair played love interests Jenna Rink and Matt Flamhaff in the movie, which centres around a 13-year-old (Jenna), who dreams of being popular, and wishes she was 30 after being humiliated by her classmates at her birthday party.
It turns out her birthday wish actually comes true and she finds herself suddenly 30 years old and in 2004, uncertain how she got there.
Mark posted his reunion with Jennifer on Instagram as it happened, and fans were quick to assume that a sequel to the 2004 flick was in the works.
Mark's caption reads: "Reconnecting with an old pal. Anyone know where we can get Razzles in Canada?"
And speculation about a potential sequel started almost immediately, with one fan writing: "Waittt, 13 going on 30 part 2??"
One commenter even suggested a potential title for the imaginary sequel. "30 going on 13," they said.
A third fan replied: "OMYYYYY JENNA AND MATTY" with dozens of heart-eye emojis.
As another happy follower said: "So you CAN just turn back time?"
"30 year-olds (who are secretly 13 year-olds) everywhere are swooning. Matty and Jenna forever," said another.
We hate to be the bearer of bad news but there doesn't appear to be a sequel - or even a reboot - in development.
In fact, Jennifer and Mark are filming a new sci-fi film for Netflix together called The Adam Project.
It's not just Jennifer and Mark you'll recognise, with The Adam Project boasting a truly stellar cast. Ryan Reynolds, Zoe Saldana and Catherine Keener are all on board for the hotly-anticipated flick, too.
There are some weird links, though. In a strikingly similar plot to the 13 Going On 30, the film's plot stars Ryan as a man who travels back in time to get help from his 13-year-old self.
Together, they have to find their late father, a brilliant scientist, who is now the same age as he is in present day, and set things right in order to save the future.
In other news, Jennifer Garner also has another Netflix movie coming out next month.
Yes Day is a family-comedy in which two parents - one played by Jennifer - give their children permission to do whatever they want for an entire day.
You can check out the trailer below:
Yes Day drops on Netflix on 12th March 2021.
We'll stream 13 Going on 30 a dozen more times while we wait for The Adam Project (and pray for a possible sequel featuring Jenna and Matt).
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