There is a school of thought that declares the average pace of degenerative aging as "normal" and states that any faster degenerations should be broken out and called "disease." This is somewhat manageable at the level of taxonomy, where you are only cataloging and describing the various ways in which bodily parts and systems break down, but as a system of thought it falls down badly once you have the ability to look under the hood to see what is going in our biochemistry. All of aging and age-related disease descend from the same collection of damage-causing processes, which like rust in a metal construction can lead to any number of different forms of ultimate structural failure - but all stemming from the same root causes. So trying to draw a dividing line between aging and disease produces issues and unnecessary additional work, especially if the researcher is trying to treat only "disease" but let "aging" progress, as you can see from the opening paragraphs in this paper: "Aging of the musculoskeletal system starts early and is detrimental to multiple functions of the whole organism, since it leads to disability and degenerative diseases. The age-related musculoskeletal changes are important in medical risk assessment and care because they influence the responses to treatment and outcomes of therapy. ... There are two major problems that one faces while trying to disentangle the biological complexity of the musculoskeletal aging: (a) it is a systemic, rather than 'compartmental,' problem, which should be dealt with accordingly, (b) the aging per se is neither well defined nor reliably measurable. A unique challenge of studying any age-related process is a need of distinguishing between the 'norm' and 'pathology,' which are interwoven in the aging. When another dimension is added, namely genetics underlying the system's functioning, even less is known about this aspect, and attempts to decipher genetic relationships between the system's components are few. ... To disentangle the aging-related pathology from the homeostasis particular for aging steady-state, is a challenging task. Despite the multiple definitions of the aging process were proposed, there is no single agreed upon and reliable measurement, therefore underlying molecular mechanism of aging is still not fully understood. The definition of aging is complicated by the occurrence of various diseases that modify body functions and tissue structures; these disease-related changes that are common in older persons are often hard to delineate from the aging process per se."

Link: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3429074/