Gainax actually said something about how because all the villages in TTGL were underground their ages are all slightly off by our standards (like, they couldn't see the sun so the way people in the TTGL universe kept track of time was different), so Yoko is closer to 16, which is still pretty young BUT not quite as bad.
Well, fair enough. So do they use different units of time than years or different years, or is it just like The Matrix where they really don't know what time it is, and therefore can't tell when days have passed.