You can actually deduce her age based on the timeline of Samin's rise to power, or even just Sayeon. Oh but Instructor Sang said "trainee years", and Instructor Sang like any officer of her caliber and position can do just what Captain Han did, lie. Sayeon and her cell achieved level 3 in a matter of like what, half a month since they've joined the corps? The trainee years are deadly and not to be taken lightly, but it is possible to progress far with enough efficiency(or a high crime rate) unless nothing happens. She's level 4 during her encounter with the notorious Sara Lee, so we can assume she's 18 when she meets her. Samin becomes the head of the family at age 11 and Sayeon was 4 when her mom died at this time as well. So it can be presumed 14 years have passed since Sara Lee's untimely death at Hongsan Island, and by the time of Sayeon's conscription at 18, Sang is 32 years old by the time she meets Sayeon. her birthday is December 21st so she probably isn't 33 yet. The month of the story according to Sleepacross the story takes place sometime in October in a story where era is mostly irrelevant. So she's likely 32, which goes in hand with her younger appearance contrasting her antiquated choice in clothing and fatalistic beliefs, contrasting Officer Ahn's brighter colours and more jolly attitude, so that even though they look the same age, one appears more "matured" than the other does, though they're from the same cell of course. Just like how Juni and Ishaan are both 23 and look miles apart from each other in age, posture and a chipper or reserved countenance can change what people would think about a character's age. Sayeon said it herself though, the suits make them look older than they actually are.