The Legend of Korra

On Airbending: "The "Avatar Extras" stated that all Air Nomads are airbenders. This would make the Nomads the only group with every individual being a bender. The creators also explain this is due to bending coming from spirituality."

it was also mentioned in the series, by the way.. once you're born, you're brought to different temples based on your gender and then you are assigned to a monk who is in charge of you as a guardian... in aang's case, gyatso.

also just a side-info, more benders are born during their respective seasons, like more firebenders are born, with the avatar having a higher chance to be born in the fire nation or i guess in korra's more globalized world, fire bender parent/s in summer.