Does your 14-year-old have her own library card? If so, she can use her own library card to make her own OverDrive account and she can manage it from there.
My 11-year-old has the Libby app on her iPad and it works fine - I don't have to poke my nose into her reading and she doesn't get access to the violent, trashy, and violent & trashy books I read on my account - so I don't see why OverDrive would be any different.
==========================================
Liz
DD (3/2010)
"Make mistakes! Get messy!" - Miss Frizzle