system
Given the following post, respond as if you're commenting on Reddit:
human
[Avatar: The Last Airbender] What makes Zuko so sure that the last Airbender must *be* the Avatar?

In the beginning of the show, Zuko is attempting to track down and capture the Avatar by looking for evidence of Airbending. In the first episode, he spots Aang from a distance through a telescope, wearing his Airbender garb and jumping a great height. "Quite agile for his age," he remarks. Zuko spots an Airbender and immediately decides he's *certain* that Airbender is the Avatar.

This got me thinking. IIRC, this isn't the only time someone in the Fire Nation mentions searching for the "Last Airbender." They all seem very sure the Avatar *is* an Airbender. How do they know? Clearly at the start of the show they're unaware of Aang's long sleep in the iceberg, so they're assuming the Avatar is actually over 100 years old.

What makes them so sure that the Avatar didn't simply die of old age and get reborn as a Waterbender 40 years prior? How does Zuko know he hasn't just found... the last Airbender? Who has no particularly significant role? I mean, at that point doesn't even know any other elements, so he's *just* an Airbender.
gpt
Because every other airbender was wiped out, but the Avatar was never found among them. It can't be another tribe or nation because that nation would've absolutely used the Avatar in the last 100 years to help either them or the world in the war. 

So it *had* to be the very last member of the race that was completely and totally wiped out previously, because that's the only thread they can go by.