[from Joel] As I get older it becomes more and more difficult to motivate myself to do things. Getting out of bed in the morning is a struggle. There's no depression associated with this, just a "what's worthwhile?" If you think about life you see that it consists of various periods of different motivational level. The motivational level is probably fed by hormones not intellect. It is logical that evolution should have made it this way. People who are unmotivated have difficulty reasoning their way out of the condition. It usually take drugs that mimic brain hormones. Nature doesn't find it efficient to keep too many old people around and fixes it so that they go quietly.
The question "why bother?" has various answers that are hormone driven. The responses "I'm hungry, I'm horny. I'm sleepy. I'm cold. I'm wet." are obviously not intellectual. However, the solutions to these "problems" call upon the brain. As we grow older, accumulated wisdom provides us with solutions, but we may lack the initiative to actually care enough to be motivated to implement them or even convince others to implement our ideas. This is probably why people speak of the elderly as "mellowing with age." It seems to me that one of the beauties of religion is that it provides a stimulus to older people just when their hormones (or lack thereof) are telling them that none of it really matters. Fear of the unknown is a great motivator.