Although reluctant with the plan at first, Aang accepts to have everyone think he had died and his remaining allies attack the Fire Nation's capital, but are thwarted by Azula.
During the finale, finding himself on a strange island, Aang is reluctant to actually kill Fire Lord Ozai, despite his four previous past lives (Roku, Kyoshi, Kuruk, and Yangchen) convincing him it is the only way.
As the Avatar, Aang controls all four elements (water, earth, fire, and air) and is tasked with keeping the Four Nations at peace.
While he did not feel the effects for many years, after he entered middle age in his 50s, the strain of this exertion increasingly weighed upon his body.
Ultimately, it resulted in Aang dying at the relatively young biological age of 66, in 153 AG.
Later, in the Fire Nation capital, Aang is seen beside Zuko, the new Fire Lord.
The series ends with Aang and his friends relaxing at Iroh's tea shop at Ba Sing Se, where Aang and Katara share a kiss.