T.S.
I nannied over the summer when I was 17 (and again at 22). The boys were 1 and 4. I took care of all the boys' day time needs, planned day trips, taught them poems and got the older ones behavior under control (he had been known as a bit of a hellion). At 22 I took care of two boys aged 7 and 11.
I wouldn't hold her age against her EXCEPT that at that age some of her decisions might not be her own. For example, with the family when I was 17, at the end of my summer with them they were headed to Spain for a month. They asked me to go with them to care for the boys (not something we talked about when they hired me for the summer) but my mom wouldn't let me miss the start of the school year.
Good luck with this.
T.