That's a pretty difficult question and I don't believe there is any right answer. My daughter is 15 but I have to say I have never measured her ass :-)
Basically it depends on your general body weight and frame, my daughter is bigger boned than me so weighs more, but looks thinner (if that makes sense).
I honestly don't think you need to worry about your ass as long as you keep healthy by eating the right foods and exercising regularly. I am sure it is all in proportion to the rest of you, and don't forget that at 14 your body is still changing.
However, exercise is always a good thing, so I have added a video that you might like to take a look at which can tone your backside.