Probably somewhere in the 10/12 year old range
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Once, Odin overheard the boys, playing at some game in the courtyard below him. He had stepped out of his study for some air, and they had no way of knowing of his presence.
They leapt at one another, each testing their strength, learning each the other's failings and moving to take advantage of them and gain superiority.
One day, that knowledge would be put to better use. They would each know the failings of the other better than their own, and they would move into that place to protect one another.
He'd seen it before, in a mock-fight between his sons and several of their companions. He'd watched, proud behind a blank mask, as his sons had drawn nearer together. He'd watched their movements, Thor's, sweeping and powerful and Loki's, moving around them and under, darting and pointed.
Loki had manifested a strong ability with magic, not unlike Frigga's save in the concentration with which it had come upon him. He was learning its control and he often chose – against the directions of his instructors – to employ it in his training. Frigga encouraged him. She hadn't grown here. She did not know how such a thing was sure to be accepted. Or if she did, she ignored it. Odin wondered sometimes, when he could spare mind from his duties to think on it, whether Loki should have to answer for that to his peers, or if they would respect the strangeness in him due to his blood.
Already Loki was prone to tricks. It was a rare day that went by without somewhere a crash or a scream followed by someone's shouting after the younger prince and his sharp answering laugh.
Thor's games kept Loki from his tricks and Loki's questions kept Thor from following the heady glow of his own strength. They each kept the other from their chosen mischief.
Odin wondered at the state of either realm should these two boys have grown apart from one another and he deemed his actions a great service to the Nine, bringing them together. They complimented one another well, both in temperament and skill.
There was a shout from below, followed by Loki's laugh.
An ache radiated from the sight of an old wound.
Odin remembered Laufey's cunning and the echoing of his laughter against the black stones. And for a moment, the laughter below him was not that of his son.
The memory of his own father's voice as his body faded into the snow, forgotten after so many years, reverberated in his mind.
Shaking off the sudden chill, Odin turned and went back to his duties.
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If you really like things chronological, and you've been reading all these updates with those of 'A Little More' as I published them, then chapters 10 and 11 of 'In the End' should come next.