Give it time, respect her.  Maybe she is confused and doesn't know what she wants at the moment.  If you want to be a good boyfriend respect her wishes and don't have sex.  If you truly love her you won't need sex.  Don't worry when she is ready she will let you know.  

Old enough to be able to take care of yourself, by seeing a doctor to make sure your healthy and also you are responsible enough to suffer the consequences if you were to have a baby.