Ellie is 14 years old in The Last of Us TV show, but seeing as the events of Season 1 take place over roughly a year, she is close to being 15 by the final episode.
In comparison, Bella Ramsey, who portrays Ellie on the show, is 19 years old in real life.
Ellie is the same age in the video game as well as the TV adaptation. However, her year of birth is quite different.
In The Last of Us game, Ellie was born in 2009, just six years after the Cordyceps Brain Infection caused a pandemic.
Whereas the main events of the show take place around 2033. This means Ellie was likely born in 2019 rather than in 2009.
However, the change in her birth year is not the only difference concerning a character’s age, as there has also been a change to Joel’s age in The Last of Us show.
The show writers acknowledge this difference with an easter egg in episode 6!

Ellie’s Age in The Last of Us Season 2
Ellie will be 19 years old throughout the events of Season 2 of The Last of Us TV Show. While the first season is still ongoing, it has already been confirmed that the second season will follow the events of The Last of Us: Part II.
In this game, Ellie is 19 years old, meaning that despite the current age gap between the character in Season 1 and Bella Ramsey, the English actress will be the perfect age for the second season.
The next season of The Last of Us will also see several other characters return. This includes Joel Miller and his brother Tommy, whose actors will also likely reprise their roles.