Here's how we know:
* The Hunger Games: Katniss is 16 at the start of the first book.
* Catching Fire: The events of this book take place a year later, making Katniss 17.
* Mockingjay: The events of this book occur over the course of a few months, so Katniss is still 17 by the time the book ends.
It's important to note that in the *Mockingjay* film adaptation, Katniss is portrayed as older due to Jennifer Lawrence's age. However, in the books, she is still a teenager.