There is a pretty good discussion on this topic in chapter 2 of Geriactric Medicine - An Evidence Based Approach (4th ed) by Cassel. This is the main reference for the info below which can hopefully add something to the answers already given.
In terms of views on ageing, there's evidence to support both:
- general principles that may apply to it; and
- it being a consequence of a collection of degenerative processes (this is apparently the more supported view).
Since almost all biological systems in the body degenerates with age and this happens seemingly at random, it's been difficult to identify particular catalysts that cause this. Consequently, biologists apparently steer away from a general theory or mechanism.
However, there are two classes of theories that have been floating around. That is 'loose cannon' and 'weak link'.
Loose Cannon encompasses theories that support the 'wear and tear' proposition. Two popular theories under this banner are free radicals and glucose.
Weak Link suggests that particular physiological systems are vulnerable during senescence and if a system fails, the whole body begins to decline. It's suggested that the neuroendocrine and immune systems are particularly vulnerable.
There is also a limit on the ability of cells to replicate - this is called the Hayflick Phenomenon (or limit). The reduction of the enzyme Telomerase, which lengthens telomeres during mitosis, is implicated in limiting a cell's ability to replicate indefinitely.