User blog comment:Pigzillion/How come Killua learnt Nen so late?/@comment-26870630-20150809014335/@comment-5780397-20150809100507

1) Though it is most likely a plot hole, we could imagine that the Zoldycks have told Killua everything on their abilities when he left Greed Island to pass the Hunter Eam: since he got the yo-yos, he might have met them.

2) I think he considered it a magical power, but he might have sensed something strange when his relatives trained their aura abilities, so he might have gotten used to it: in fact, he never showed any signs of surprise when he was introduced to it.

To answer the last question, Killua may have proven himself to be skilled in ALL fields of assassination, so his training in Nen was postponed; on the other hand, Kalluto was weaker than Killua in combat, so his training began earlier. Or, who knows, he might have asked Illumi to teach him, or learned it all by himself.