Children do not count as people, because they are too likely to die in their world.
Seriously. In their world, they do not count a child as viable as a person until closer to twelve-fourteen, since the death rates for kids under that age is too high. (Though, to be fair, most children that don't make it die before the age of five.)
They have magic, but high magic and well performed medical techniques of any sort are roughly in the same range as those in our world's (Western world) 18th century.
That's also why Anders is creeping up on manhood, even if he is only twelve.
*Much like "gosberries" are not "gooseberries" in their world, there are different rules for some things involving time, dates and so on. A lot of it is rough, compared to our way of thinking. For instance, while they count the days, a birth-time would be the season you were born in, not even the month.
So, a man who is in his thirties would have "seen" two decades of life. Even at that, it could be pushed up or back, depending on how old they seem. A childless woman of twenty-five, who's small and lithe might be said to be in her first decade, even if it literally isn't true.
*This probably won't be explained in the next book, so I did it here, in case anyone cares. There is a lot more information that I have about the worlds being looked into than ever makes the page. It's like that for everything I've worked on.