In my little research I discovered that it's up to debate. Yoko is described as having the same age as simon, which can mean both exact (14) and age group (teenager).
Given that nia looks close in age to simon and treated differently from yoko (though this maybe due to status); also that after timeskip yoko has the same build as before, but nia and simon changed drastically; I believe Yoko is 16-17 pre-timeskip, which matches her body development.