In Cruel Intentions (1999), the main characters are written as teens, mostly around high school age, with one key character explicitly described as 15. The actors were generally much older than the characters they played, which is part of why the movie’s age gap feels so noticeable.

Character ages

Character| Approximate age in the story| Notes
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Sebastian Valmont| Late teens| A prep-school student; the film frames him as a teen 27
Kathryn Merteuil| Late teens| Also a prep-school student and Sebastian’s step- sister 27
Annette Hargrove| Late teens| A teen peer in the same social circle 27
Cecile Caldwell| 15| Explicitly described as 15 in discussion of the film 7

What matters most

The movie is basically a teen drama, so the core cast is intended to be in high school age rather than adults. The biggest standout is Cecile, who is specifically treated as 15, while the others are presented as older teens.

Cast age difference

A lot of viewers notice that the real-life cast was older than the characters. For example, Selma Blair was 27 during filming, while the role she played was meant to be much younger.

If you want, I can also list the real ages of the main actors in 1999 next to their characters.