Just because it's Thracia 776 doesn't make Leaf 16.
He could be 15 and not have had his birthday yet, making him at least 16 by the end of FE4.
And, Celice is two years older than Leaf, making him 17 at the start of Gen two.
If Levin hadn't been to Isaac yet, that doesn't mean that gen two STARTS during 776.
Celice has to fight his from Sophara all the way down to Isaac, which encompasses the year from 775 to 776.
Heck, he meets Levin in Ganeshire, which means Levin went there, left to see August, and then went back to Isaac.
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