Do dogs get aggressive as they age?

I have a chow/german shepherd mix who is about 3 years old. Lately she has been snapping at us trying to bite us. We try to pet her when shes laying down, looking right at us and wagging her tail and she barks and tries to bite us. Also she has started barking aggressively at visitors. I heard later on in life they start to lose their sense of smell and become violent but I figured she was still pretty young. What can we do? Is this normal?