How old was Avatar: the Last Airbender's Aang when he died? The iconic animated hero may have had an eventful life after the conclusion of his own series, but in the opening moments of Avatar: Legend of Korra fans learn that he met his death in an unexpected way and at an unexpected age. Bryan Konietzko and Michael Dante DiMartino, the co-creators of both Avatar shows, could not have made the decision of killing off such a central character lightly.
As depicted in the original Nickelodeon show, the twelve-year-old Aang is presented as a compelling, complex character. A pacifist at heart, Aang's sense of duty often pulled him toward battle and destruction and while he is shown to be fun-loving, that was also balanced by his enormous capacity for empathy and guilt. Aang's narrative arc following the reluctant hero to his final and world-changing showdown with Fire Lord Ozai is one of the best in animated show history. So when fans come to learn that Aang died off-screen and at a relatively young age there was a sense of anti-climax. However, when considering just what the two series have to say in terms of corruptive power and the violence assumed by war, it is possible that Aang's early death was as poetic as it was premature.
From what is acknowledged in The Legends of Korra (and the Avatar comic series which fills in some gaps within the timeline), the audience learns that Aang entered a relationship with Katara, his friend and steadfast ally from the original series. Eventually marrying, the couple had three children: the non-bending Bumi, water-bender Kya, and air-bender Tenzin. After many decades of building a new, more peaceful world, Aang begins to feel the effects of being frozen in ice for a century, beginning to feel his life force wane as he entered middle age. The strain became too much to bear and Aang set his affairs in order and died peacefully in 153 AG ("after genocide," referring to the destruction of the Air Nomads), at the age of just sixty-six, leaving his family and reincarnating into the new Avatar, Korra.
Of course, while 66 accounts for Aang's biological age at the time of his death, one mustn't forget the additional one hundred years he spent in the ice prior to the events of Avatar: The Last Airbender. Born in 12 BG ("before genocide"), Aang's overall age is actually one hundred and sixty-six years. Spanning so many decades and witnessing such monumental worldwide change, Aang's perspective and timeline are unlike any other character in the universe and that proved instrumental when leading the Harmony Restoration Movement in the wake of the Hundred Year War.
So while Aang's death may feel untimely, the relatively peaceful manner in which he goes is very much in line with his pacifist leanings and certainly preferable to a more violent end. Surrounded by family, friends, and a world which he saved and helped to rebuild, there is a certain beauty in the simplicity of it, which is almost reminiscent of the Air Nomad's monk-like teachings. And, of course, with the creation of Avatar: The Legends of Korra, there is a narrative necessity in setting the sequel in a post-Aang world. With the in-universe mechanics surrounding Avatar reincarnation, it is necessary for the old Avatar to die in order to bring about rebirth. And regardless, a continued over-reliance on Aang's story would ultimately have robbed the audience of the opportunity to fully committing to Korra's journey.
Noted for the inclusion of adult themes in what is ostensibly a children's cartoon, with plots focused on death, legacy, shame, and loss as early as The Last Airbender pilot, the show's creators are no strangers to taking big narrative swings. With the decision to end Aang's earthly life early and somewhat unceremoniously, the show creators could forge ahead in a new direction as the story re-centered on Korra. Simultaneously reinforcing the original series' assertion that the balance between life and death (and one's ultimate acceptance of that truth), choosing to kill Aang at a relatively young age manages to bring new poignancy when re-watching Avatar: The Last Airbender.