I think the thing with the kids could be explained by the fact that they have already been alive for 16 years before they are available to control. Plenty of time for them to be sent off to the capital for a bit of education, training etc. The captains that are adopted are a little trickier to explain. I like the idea of the ceremony, or being kitted out with the funky armour and the horsemen, and that they have to go to the capital for this to happen.
I do get a bit confused as to where the captain is coming from however, I don't know what his original unit was. Are they just disbanded or do they live on with a new captain?
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