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The Yangwu avatar was discovered by the Southern Air Temple prior to the Air Nomad genocide, but his personal identity had not been revealed to the world at the time of the Air Nomad Genocide. What became of the young Avatar is not known. Most people assume he was killed, or escaped but lived an ordinary life in obscurity with the scattered survivors without having ever been told of his true nature.
Some people who use the Avatar Era for writing dates continue to extend the Yangwu era indefinitely, absent any information about succeeding Avatars. If the Riwu Avatar is still alive, he must be quite old, about 107 years as of 95 AG.
Artist's conception
See also Aang on the Avatar Fandom wiki, for how the Yangwu Avatar's life played out in the show canon.