I think you're wrong. Some do, but majority stays the same or even lose weight drastically. People over 55 have in average three chronical deseases which usually means different combinations of pills and stuff and they usually get skiny because of interactions of different chemicals in metabolism. It of course depends on their metabolism and lifestyle. Old people are usually devided in two groups: active and passive. Passive are those who decide that afret retirement there is no use of them anyway and don't do much, they usually lay on couch all day long and expect to be taken care of. Those get fat or dye from other reasons pretty soon (few years). Active are the ones who always want something to do, togo for walks or to play cards and are never bored. I'm spending quite a lot of time with old people latey in home for the aged and in the hospital too and I came to the conclussion that much more active old people.
Ok, I'm way of topic now...
I don't have much time to work out. I do have a very dynamic lifestyle lately tough, I run around in from one side of Ljubljana to another dayly trying to get from one clinic to other and to work and to school... I try to go to the hills once a week too.