They had to make sure that whoever they hired wouldn't sing "16 going on 17" with a deer-in-the-headlights look on her face.
I thought she could easily pass for 16. I think when people knew what her real age was, they think of that.
I used to think that. However, having seen some of the secondary school aged girls these days, who look 16-going-on-27 and have the confidence to match, I think she seems positively babyish :-)