16 is probably the age where parents have to start paying the 100,000 tax for their kids, thus a lot of 16 year olds end up as slaves when their parents can't pay.
... I got that 100,000 number from Ch. 14, but apparently it's only 30,000 now? Or perhaps it's 100,000 for the head of house and 30,000 for additional members? That would certainly make it more reasonable.