why do older dogs lose their teeth

Regular dental care can help your dog keep her teeth well into adulthood. Schedule an oral cleaning with your veterinarian every six months, and ask your vet what you should do at home to care for your dog's teeth. Chewing can also help, because it mechanically removes debris. Give your dog rawhides and other chew toys.

Hard food is preferable to soft food, so avoid giving your dog only wet food. Dietary changes can also make a difference. Fillers such as corn and wheat may cause health problems, particularly oral health problems, in some dogs, according to veterinarian Tom Lonsdale. Finally, consider giving your dog an occasional raw bone to chew on.

Raw bones are safe for dogs; it's cooked bones that pose a risk, and chewing these bones can help prevent tooth decay.
"Normal" as in it happens to most dogs, yes. "Normal" as in it should happen as a natural part of aging? No. Not the way it happens with the average dog, anyway.

Rotting teeth, and diseased gums that aren't capable of holding onto the teeth is a direct result of improper diet and the total lack of stimulation the average diet of processed foods can provide. Genetics also play a big role, and poor breeding practices really compound the problem in many lines of purebred dogs.

It's not seen as a problem that should be addressed in one's breeding program, though, because gum disease and tooth loss is seen as "normal". Dr. Tom Lonsdale discusses at length the state of pet dental in his book, "Raw Meaty Bones". Really interesting is his take on what is supposed to happen to the teeth and gums naturally among carnivorous species, and the role tooth decay plays in the life of individual animals, as well as the health and dynamics of an entire pack in the case of social carnivores that hunt together.

Worth a read even if you're not particularly interested in raw feeding.