The jawline changes in a characteristic way. The area just in front of the jowl recedes, probably because of ligaments that hold the skin onto the bone. It is soft tissue that changes, not bone. I still do not know if the jowl formation is a descent-phenomenon or a simple expansion, or both. The soft tissue gets thinner behind the jowl. There is a bony notch on the mandible just behind the jowl that becomes visible. The whole jawline varies tremendously with facial weight.
This animation is of a woman from 23 to 63. Look at the lips. Look at the characteristic shape of the jawline: it recedes posteriorly and makes a bump at the jowl (which may be from either new fat in that position, or loss of fat around it). Also, the characteristic hollow anterior to the jowl.
Look at this woman from 16-years old to 68, you see the same changes as in the patient above.