To answer some of your questions, yes, they are just like any other mammal, including humans, their bodies start to deteriorate with age. My oldest glider, Aramis, for example, is showing clasic signs of age. He has a whiskered graying look to his face, his head is flattening on top, he is gaining weight as he becomes more sedentary with the difficulties of an old man trying to climb in his pouch and around the cage. He will be here with me until he leaves this earth because he has gotten old and cranky, his eyesight is not as good as it used to be, so he tends to attack if something gives reason to defend himself or his cagemate, whether real or imagined. He is still adorably cute and I love him just the way he is.
With that being said, just like people, some do not show their age as readily. His cagemate, Hannah, is showing an older look, but she is still quite trim and a little more active than he is. She does not see as well as she used to, so she must be approached with caution, but usually she is okay when she smells that it is me.