Nathan was taken as a young child, less than 10 years old.
I often thought about the 1 gifted heir per Rahl, and it really makes little sense, the line would die out with just some bad luck eventually. And with the Rahl like being the MOST important line in the gifted world (immunity to dream walkers), it makes less sense that they can only have 1 gifted heir, though I do think that Goodkind states that Panis Rahl was not gifted...