It's their first night on Ember Island, when the house is too big and they light a fire on the beach and splash around in the water to their hearts' content, that Zuko notices.
It's easy to forget.
Aang carries the weight of the world on his shoulders around with a smile and learns the elements all at once, bit by bit. He talks to spirits and cuts down countless Fire Nation ships with a slice of his arm through the air and has impossible power wrapped up inside him. He smiles at Katara with that bashful in-love look on his face and laughs with Sokka and grins at Toph and is so incredibly carefree— prancing around with all the grace of a light-footed twelve year old. And it hits him, all at once, that the Avatar- that Aang- is twelve. A child.
He laughs with an air of innocence that's out of place in the world, and a child has to kill the Fire Lord.
"It hits you sometimes, doesn't it?"
Zuko tenses at the sound of Sokka's voice (he's usually better at hearing people approach) and doesn't like the lack of humor in it. He doesn't have to ask what he means.
"Yeah." he replies quietly; Aang laughs and freezes the water around him, "He's twelve. So is Toph."
"You're sixteen," there is no accusation, just grim truth.
"You're almost sixteen."
"And Katara is fourteen." There is a bitter smile pulling at his friend's (friend's?) lips, unnatural and all wrong.
Zuko huffs a harsh, cracking imitation of a laugh, "And we're supposed to save the world."
Aang laughs.
Zuko wonders if he will laugh before he goes to face the Fire Lord, if he will smile and walk determinedly to his grave. He pushes the thoughts from his mind, because all they can do is train him, prepare him as best they can and hope to the spirits he'll walk out of it on top.
And Aang laughs.