My dogs teeth are falling, help ?! Details inside!?

Shes about 9 years old, shes a black toy poodle. We feed her cesars dog food everyday (half of little box package thing) with pebbles. i know its not good, but she bites us when we try to brush her teeth, so we dont do it often. and right now on of her front teeth just fell out. theyre somewhat small besides the fangs or whatever theyre called, but is it because of old age ? infection ? her teeh are kind of brownish at the top of them.