The storylines on Sharp Objects shift between the past and the present, often revisiting Camille's childhood to explain her behavior as an adult. The trauma she suffered as a teenager is directly connected to her unhealthy coping mechanisms now, and it's all been brought to the forefront upon her return to Wind Gap. But it's difficult to pinpoint exactly when all those timelines took place, and how many years have passed. Determining how old Camille is on Sharp Objects could be helpful in figuring it all out.
Camille's age isn't explicitly stated on the show, and she dances around it when asked by Amma and her rollerskating posse. Actress Amy Adams is 43 years old but she might not be playing her actual age as Camille, because the other actresses on the show don't seem to be playing their actual ages either. Patricia Clarkson is 58 years old, so if Camille was in her forties, then Adora would only have been 15 when Camille was born. Adora was a young mother, but the Gillian Flynn novel that the show is based on stated that she was 17 when she had Camille.
Eliza Scanlen, who plays Amma, is definitely playing younger than her years. She's 19 years old in real life, but Amma could be anywhere from 13 to 15. She's only 13 in the book, but it's less clear on the show; Camille mentions that the seventh grade was only a year ago for Amma, but Scanlen told Refinery29 that Amma was 15.
While Scanlen is older than her character, Amma has also been aged up from the books. The same might be true with Camille: she might not be Adams' exact age, but she could also be older than Camille was on the page. However, Camille didn't state her age in the book, either. From context clues, one could gather that she was in her early thirties. In the novel, one of Amma's friends guesses that Camille is around his mother's age: 33 or 34. Camille notes that that was "close enough," though she knew the boy's mom from high school, where she was "a few years older" than Camille.
That would mean Camille was around 30 or 31 years old. In the book, Amma was born when Camille was already away at college, which accounts for how little she knew about her younger sister's life. Their age difference was big enough that they didn't grow up together like Camille and Marian did. That appears to be the case on the show as well, but their age difference might be bigger if Camille is closer to Adams' age.
There are some background clues about Camille's age as well. In the first episode, a George H. W. Bush 1992 campaign sign can be seen on one of the Wind Gap buildings while young Camille and Marian are rollerblading. Camille was around 13 when Marian passed away, which would make her 39 years old if the current timeline is set in 2018. However, a 2008 Obama poster can be seen in Camille's apartment in the same episode. If even the current timeline is a decade back, then Camille would be closer to her early thirties age from the book.
Camille's age is not definitively stated in the first five episodes of Sharp Objects, nor is it listed in the book that inspired the show. There are only a few hints to go off of, and even those can be conflicting. Ascertaining how old Camille is doesn't result in a satisfying answer, but one can assume that she is in her thirties at the very least. Unless she has another birthday in the final few episodes that emblazons her age on the cake, that's the best guess.