Thermodynamic Models of Aging in Living Systems
It is remarkable that closely related animals, with very similar genetic sequences, can have significantly different lifespans. Generally speaking, dog proteins and human proteins have only a few amino acid differences (therefore limited genetic differences), yet a 14 year old dog is geriatric (with cataracts and gray hair) while 14 year old humans are only reaching sexual maturity and are several years away from their physical prime (and many years away from being geriatric).
Living systems (which we will term the "Primary machine") are complex molecular assemblies that are only able to function if such organization is maintained at some critical level. The primary machine, as a complex assembly, is entropically disfavored, and subject to spontaneous disassembly (decreptitude) over time; however, the expenditure of energy (i.e. work) can oppose entropic forces and thereby maintain the necessary organization to permit acceptable functionality.
Assume that the utilization of energy to perform the work necessary to maintain the primary machine is provided by another (albeit perhaps less complex) molecular machine. This "Primary repair machine" is itself entropically disfavored, but can also potentially be maintained through energy expenditure. There are different models by which this might be accomplished.
Model 1: The Primary repair machine is maintained via a Secondary repair machine and this "Secondary repair machine" is distinct from the Primary repair machine
Such continuum of sub-levels of repair machinery could go on and on, ensuring the original living system with a very long functional lifespan. A loose analogy might be a car. After 40,000 miles (maybe 4 years) the tires need replacing. The repair shop uses a tire machine to replace and rebalance the tires. The tire machine is good for 10 years of use before it needs to be repaired/replaced. The tire machine is housed within a garage (so that it is protected from rain, etc.). The roof on the garage is good for 20-25 years and then needs replacing. And so on.
By analogy, different levels of repair machinery that maintain the Primary Machine may exhibit their own lifespan (with the possibility that progressively lower levels of repair machinery have longer lifespans due to lower required activity - i.e. "slower metabolism")
However, things are also evolving; importantly, progeny are being produced, some with improvements over the original system. Thus, maintaining the Primary Machine over time, while possible, might be futile at some point (i.e. provide no advantage) when it is outcompeted by a more efficient progeny (even though the Primary machine may be running perfectly adequately according to the original specifications). In other words, repair machinery several levels deep are practically not needed because the original design will, at some point, be obsolete (and outcompeted).
Using the automotive analogy, machinery to adjust carburetors or ignition points, while capable of keeping cars with such technology in working order, are no longer needed with newer cars that have been designed with (more efficient and reliable) fuel injection and electronic ignition. Similarly, equipment needed to keep internal combustion engines in general running are no longer needed upon the changeover to (more efficient and reliable) electric vehicles. Thus, there is no need to develop "deep level" repair machinery (which can ensure long functional lifespan) for an overall process that is subject to technological improvements (i.e. more efficient mutations). The level of repair machinery that is developed (selected for, or practically needed) may be a function of the generational time and mutation rate.
Model 2: The integrity of the Primary repair machine is also maintained by the Primary repair machine
In other words, there is only one repair machine (the Primary repair machine), and it is responsible for maintaining the integrity of the Primary machine, as well as itself (there is no secondary repair machine that maintains the Primary repair machine). This system is now keenly dependent upon the integrity of the Primary repair machine - once it suffers a fault that is not caught and repaired, then repairs upon both the Primary machine and itself (the Primary repair machine) are negatively affected (and would continually spiral downward). This design requires fewer different types of machines, and is therefore potentially much simpler. The Primary repair machine would also be more general in its function since it must effectively service two different machines (both the Primary Machine and the Primary repair machine. Lifespan of the Primary Machine is now keenly dependent upon the functional lifespan of the primary repair machine.
Model 2 might be more akin to the situation in living systems. In this case, one element of the Primary repair mechanism is protein quality control. The quality of protein quality control is dictated by the quality of the protein quality control (!).
To improve the lifespan of the Primary Machine, the functional lifespan of the Primary repair machine must be increased. In consideration of Model 1 (above) it may be feasible to introduce some novel designed Secondary repair machine - whose purpose is to maintain the integrity of the Primary repair machine.
"I could be reworked, but I'll never be top of the line again" (Bishop, Aliens).