Sorry for resurrecting an old topic, but I just had to get my thoughts out there...
I think it's similar to how mutant powers are explained in the first X-MEN film. They just happen to use their powers at really emotional times. Like in The Boy in the Iceberg, Katara goes off at Sokka and causes the iceberg to break up without even meaning to. Naturally, these "emotional times" would usually occur between the ages of 10 and 15, but could occur earlier. So, if you get really angry and someone taps your shoulder and says "did you realise your house just caught on fire?", you're a firebender.






IT'S AANGSTIN POWERS! 








