The reliability-engineering approach to understanding aging is based on ideas, methods, and models borrowed from reliability theory. Developed in the late 1950s to describe the failure and aging of complex electrical and electronic equipment, reliability theory has been greatly improved over the last several decades. It allows researchers to predict how a system with a specified architecture and level of reliability of the constituent parts will fail over time. But the theory is so general in scope that it can be applied to understanding aging in living organisms as well.
In reliability theory, aging is defined through the increased risk of failure. More precisely, something ages if it is more likely to fall apart, or die, tomorrow than today. If the risk of failure does not increase as time passes, then there is no aging.
Reliability theory is, like evolutionary considerations of aging, a "why" theory rather than a "how" theory. It is useful as a reference point and foundation when thinking about how to address the aging process via medical science, and fits well with Aubrey de Grey's Strategies for Engineered Negligible Senescence. You can download the PowerPoint materials at Leonid Gavrilov's website.