"Emmy Rossum is 35. Why on Earth is she playing Tom Holland's mother??," one person wrote.
"I do like Emmy Rossum but why hire a 35-year-old woman to play the mother of a 25-year-old? Why not hire an older actress? Just a thought!," wrote another.
"Kinda strange since she's 10 years older but if they have Tom play a 20-year-old then I can see it I guess," pondered one user.
"Whatever casting director thought Emmy Rossum looked old enough to play that grown man's mother needs to be fired," wrote another.
The casting decision has been labelled sexist and ageist, and a reflection of how older women are overlooked in Hollywood.
"A middle-aged woman losing out again. They are invisible on screen," wrote one Twitter user.
Al Jazeera shared the news alongside the fact that "older women are dramatically under-represented in Hollywood".
"In 2020, men over 40 were almost 2x more represented in film than women the same age," the media outlet said.
Writing for The Cut, Olivia Truffaut-Wong said: "I think it's time for Hollywood to enact a new rule that every man who plays the son of an actress less than 15 years his senior must, in their next project, play a father to an actress at most 10 years his junior. Fair is fair."
Some have suggested Rossum will only play the role as Holland's mother in flashbacks.
"Guys — Emmy Rossum is playing Tom Holland's mom because the vast majority of her scenes are flashbacks to his character's very early childhood when he endured the trauma that caused his Dissociative Identity Disorder. It started before he was 5 years old," Carina Adly Mackenzie, who was the writer of the series Roswell, New Mexico, said.
Comparisons were drawn to Mandy Moore's role in This is Us.
Moore was younger than all the actors who played her children in present day.
Holland plays the lead, Danny Sullivan, in The Crowded Room and is executive producing alongside Akiva Goldsman.
The first season is based in part on Goldsman's own life and inspired by Daniel Keyes' biography The Minds of Billy Milligan, according to Deadline.
Sullivan is apparently loosely based on Milligan, who was the first person acquitted of a crime because of multiple personality disorder (now known as dissociative identity disorder).
Rossum will play Sullivan's mother Candy.
Amanda Seyfried is also in the series, playing a clinical psychologist named Rya.