insert that's not how Japanese law actually works reply
Technically, they were underground and didn't have timekeeping.
The TTGL wiki also doesn't list age in the profile, and the other two wikis that come up disagree on age. Based on looks, it's pretty obvious she's older than fourteen. Not just boobs – everything is well-developed.
But post-timeskip Yoko is also even better than pre-timeskip, anyway.

Insert "Different calendar because underground" justification.