If uve seen the episode ''The Warriors Of Kyoshi'', u might have noticed that the avatar Kyoshi has been born??there over??400 years ago. with a bit of calculation ull find out that avatars appearently reach an age of about 150 years! I mean, thats a pretty high age right?
ofcourse each avatar will reach another??age then the other, but still... I guess Aang will be the oldest avatar ever lived, considering he'll reach an age of about 250 years (100 years in the iceberg), ofcourse only if he wont get killed before that . Also dont forget King Bomi... appearently avatars arent the only one getting that old.
This means that there still have to be a lot of airbenders left from before the war. I dont believe they all??got killed by FN soldiers... certainly not if u remind what Iroh told about the Air Nomads: ''Air is the element of freedom. The Air Nomads detached themselfs from worldly concerns and found peace and freedom.'' They couldnt have possibly been killed all with such a need for freedom and peace...
maybe u know what i mean now