In general, terms, when a person is in advanced age, bone density and strength decreases.
That is why in a very advanced age, a person can look frail and weak.
With specific regard to the jaw bone, jaw bone typically shrinks rapidly due to the loss of teeth. However, if you keep all of your teeth in good condition well into your old age, your jaw bone volume should be maintained and shrinkage will not be as pronounced as compared to an old person who has lost all of his or her teeth.
If you have a concern about the size of your lower jaw, or if you are considering jaw reduction surgery, please seek the opinion of an Oral and Maxillofacial Surgeon, who can make a better assessment in your case.