he is youngest because the airbending elders told him early at 12 instead of 16. the reason that they did that is because they know that the moto nation was going to attack them, so Aang needed to be ready. it also should have taken him a couple years to master the elements, but that was rushed too
I think this swali means why he's the youngest.After the other airbenders told Aang he was the Avatar,he ran away because his Marafiki couldn't except him the way he was.While he was running away with Appa,there was a terrible storm.Both Appa and Aang fell in the water and Aang went into the Avatar state.While in the Avatar state,he froze them inside the water.He was in the iceberg for 100 years,making him 112 years old.
because the monks were supposed to tell him he was an Avatar at the age of 16 but because of the war they had to tell him at an early age hope it helps