There are many thematic takeaways from The Hunger Games franchise that tackle a wide range of important issues. As a young adult series, it doesn't shy away from exploring dark topics; it just does so in a way that is relatable and approachable to younger readers, but it certainly isn't limited to that age group. The original books were a massive hit that connected with readers of all ages. So it makes sense that the films would go on to become massive blockbusters as well.
However, one of the things that may not be totally clear to those who only saw the movies is just how young the characters in The Hunger Games actually are. Jennifer Lawrence, Liam Hemsworth, and Josh Hutcherson were all years older in real life than their characters were. Part of the true horror of the titular games is the extreme youth of the participants. This is also true about Katniss' younger sister Prim who is her first priority when it comes to her decision-making throughout the series. The films don't clarify exactly how old Prim is by the end of the series, but the final book reveals the truth.
Prim Was Barely a Teen During The Hunger Games: Mockingjay
During The Hunger Games, it is clear that Prim is 12 years old when the story starts. The inciting incident for the entire series is the moment Prim's name is drawn at the Reaping Ceremony, and before that, Katniss tells her not to worry about her first year because her name only appears in the lottery one time. The children of the districts must add their name to the lottery once a year from the time they are 12 until they are 18, with the number of ballots compounding over that time. Katniss elects to add her name additional times in exchange for necessary provisions but has prohibited Prim from doing the same. Despite that, Prim is still chosen, and Katniss volunteers to take her place.
So, it's clear at the start of the story that Prim is 12, and she is, therefore, 13 when Katniss is forced to enter the Quarter Quell during The Hunger Games: Catching Fire. Since those games ended in a full-out revolution, there isn't as clear of a sense of time passing in The Hunger Games: Mockingjay. However, in the book, Katniss reveals that Prim was "not yet fourteen" when she was killed in the Capitol. Everything Katniss did throughout the series was to save her sister, and in the end, all she was able to give her amounted to less than two extra years, thanks to this tragic turn of events.
How Did Prim Die In The Hunger Games: Mockingjay
Prim's death was a terrible tragedy that haunted Katniss for the rest of her life. Even though Katniss was able to get some kind of revenge for her loss, it wasn't enough to heal her. President Snow reveals to Katniss that President Alma Coin is the person responsible for Prim's death, and while Katniss is reluctant to trust him, she is forced to accept that he is right. She initially places the blame on Snow, but as he said, he and the Capitol forces had already been defeated at the point of the bombing that killed Prim, so there was no strategic advantage to an attack.
Coin, however, had a reason to attack the Capitol. It was all part of her propaganda plan to turn the citizens of the Capitol against Snow and join the revolution. The bombs deployed were also part of a plan Gale had previously explained to Katniss, which was meant to disperse the maximum amount of damage to their enemy. By having a two-stage detonation, bombs would leave wounded survivors, and then the second wave would kill the first responders. Prim was a first responder despite only being 13 years old. Due to her extreme youth, and the rigorous nature of Coin's deployment teams, it's likely Coin personally made sure Prim was part of this wave of first responders who would be killed in the bombing.
Primrose Everdeen was the catalyst of the entire series for The Hunger Games, and her impact on the characters and the story can't be understated. She was only 13 years old when she was killed in The Hunger Games: Mockingjay, and her youth only makes the tragedy that much harder for Katniss to bear after everything else she had suffered. Prim's death is a truly heartbreaking moment.