system
Given the following post, respond as if you're commenting on Reddit:
human
[Avatar] When is the next Avatar selected?

Watching Legend of Korra and it seems that the new Avatar is born when the previous one dies.  This means that there's a good chunk of time, say at least 15 years, where there is no Avatar. 

However, this wasn't the impression I got when watching Last Airbender.  It seemed to me that Aang was being trained to be the next Avatar, but that Roku was still alive and Aang fled when Roku died because he knew it meant he had to step up to his responsibilities and couldn't.

Otherwise, the Fire Lord Soujin had like 10+ years to find Aang and kill him.  Not only that but Aang was able to travel to the Earth Kingdom and hang out with Boomi. Wasn't there a war on already?
gpt
Avatars are born as soon as the previous one dies. We go straight to Aang's birth after Roku's death in episode 6 book 3.

The reason Firelord Sozin waited 10+ years after Aang was born was because he was waiting for Sozin's Comet so he could use its power to deal a deadly first strike not just against the Air Nomads, but also the other nations

Plus, he couldn't prepare and plan for his invasions when Roku was still alive, thus he needed to use some of this time to prepare.