By "signs of aging", the Ageless Animals folks are referring to the effects commonly associated with senescence and accumulated bodily damage of the sort that causes most animals to decline in health in their advancing years. It is not that these animals do not change at all as they age, but rather, that they do not tend to display some of the more pathological features seen in other animals at much younger ages. This is worth studying for the simple fact that it is very interesting to see how aging manifests in a cross-species context, but also because some of what we learn from our long-lived cousins could eventually be applied toward improving medicine for humans.
Human life expectancy worldwide more than doubled between 1900 and 1985 (from 30 to 62). This was not the effect of a sudden mutation or other drastic change in human biology, but rather, the effect of improvements in disease control, nutrition, and other socially mediated factors.
Nobody today knows for certain how far the current generation of available medical treatments can continue to increase the healthy lifespan, however, it is a common sentiment that we are probably approaching the limit of how effective our health care regimens are for the oldest old. Once the average human reaches 70 or 80 years or so, the body's protective and repair mechanisms become much less able to cope with the daily bombardment of damage from without (infections, general wear-and-tear) and within (metabolic byproducts, cell death). So even though people in their 80s and 90s can most certainly lead live rich, full lives, such persons remain far more vulnerable than younger people to death from acute illness as well as to cascading systemic breakdown. It is this vulnerability that proposed future medical regimens would like to address -- and it could very well be that some of these future regimens come into being through the study of very long-lived animals.
Perhaps one of the most impressive aspects of these "ageless animals" (and of certain other long-lived animals, such as elephants and some birds) is that they achieve their long, relatively healthy lives without any of the benefits that we humans have used to double our own average life expectancy over the past century. Turtles, rockfish, and whales do not visit doctors, or take antibiotics when ill, or depend on sewage plants and sanitary facilities to protect them from environmental ills. One almost has to wonder how long these creatures might live if they did have their own customized health programs; however, for the moment, it is sufficiently awe-inspiring to realize that these animals were living well into their second century back when we humans could barely expect to reach age 30!
Of the long-lived animals presently known, some appear to have a built in "negligible senescence" property. From the Ageless Animals site, we learn that:
Caleb Finch at USC coined the term "negligible senescence" to describe very slow or negligible aging (Finch 1990). He listed several animals with this characteristic, including rockfish, sturgeon, turtles, bivalves and possibly lobsters. Later in a paper from the first Symposium on Organisms with Slow Aging (which the Director of this project also spoke at), Finch further described criteria to test the occurrence of negligible aging. These include no observable age-related increase in mortality rate or decrease in reproduction rate after maturity, and no observable age-related decline in physiological capacity or disease resistance (Finch and Austad 2001).
This is a very important observation because it clearly indicates that the life of an animal is not necessarily inextricable from a fixed "expiration date" built into his or her deepest inner workings from the start. So while I do not deny the significance of the many human attempts over the years to garner meaning and poetic substance from reflecting upon the bodily breakdown that tends to eventually kill us all, it is well worth pointing out that some of this philosophizing is at least partially rooted in what appears to be a false belief -- that age-related decline of the sort experienced by humans is "inevitable" in all members of the animal kingdom, and that our own decline somehow "connects" us with everything else on Earth. Statistically, even the "ageless animals" will tend toward eventually being killed by accidents or other mishaps -- making it improper to describe them as "immortal" -- but there is no reason for any person to consider the mayfly more her spiritual kin than, say, the rockfish.
Rockfish studies have revealed a positive (though not absolute) correlation between longevity and depth, latitude, and maximum size. It is not currently known exactly what biological characteristics in turn correlate with these surface observations, but some data suggest that certain rockfish might be "trading off" in favor of slow growth (and therefore slower metabolism, which means less internally accumulated damage) as opposed to more rapid growth (which improves survival in predation-heavy environments, but which entails a faster, more damage-producing metabolic rate).
Moving into the realm of mammalian longevity, elephants and humans stand out as the most reliably long-lived land mammals. Looking at these two species in particular, it is quite interesting to note that while humans have a higher documented maximum lifespan (Jeanne Calment was the oldest verified supercentenarian by the time of her death at age 122), elephants can live to 60 - 70 years more reliably than humans "in the wild" probably could.
Additionally, it is suspected that if not for tooth wearout in around their seventh decade, elephants might live much longer (since the most common cause of death in older elephants is not heart disease or cancer as it is in humans, but starvation due to not being able to properly masticate food). Elephant longevity might regularly exceed that of humans if better dentistry services for pachyderms existed, probably in part owing to their large body mass (which is correlated positively with longevity within classes of animals).
The longest-lived mammals of all, however, are not land-based creatures but citizens of the sea. Bowhead whale lifespans have been calculated as exceeding 200 years in some cases using a method which extrapolates age from the ratio of left-handed to right-handed aspartic acid molecules in the whales' eye lenses. Multiple factors probably contribute toward whale longevity, including body mass, slow growth, low reproductive rates, and a cold-temperature habitat.
As far as applying rockfish and whale study results to human medicine, there do not seem to be any direct routes to this as of yet -- but the data does at least suggest that interventions which target the byproducts of metabolism may indeed be a step in the right direction. All the long-lived animals highlighted in this article have specific bodily features and/or behavioral tendencies (rockfish actually tend to seek out deeper, colder waters as they age) that serve to minimize metabolic damage. Conceivably, if metabolic damage can be addressed in humans, it might be that many of the health problems we currently associate with "old age" today will instead eventually be associated with a specific category of treatable conditions.
It is important to stress in such discussions as this that mitigating metabolic health conditions will certainly not mean that people will cease to get old, or that everyone will be granted comic-book-like superpowers once we find out how to stop oxidative (and other) damage from accumulating to the point where it destroys our organs. Longevity medicine proper is not to be found in the "fetishization of youth" or in fantasy, but in recognizing that getting old should not have to mean getting sick. It is one thing to say that people should be helped to "age gracefully" as opposed to "cling unreasonably to their youth"; it is quite another to extend this sentiment to the idea that life itself ought to remain the exclusive province of the young. Life is for everyone!
And it is also important to emphasize that longevity medicine is simply one essential component of health care as a whole, and working to help elderly people experience a less precarious existence (at least in my estimation) falls into the same ethical category as working to improve sanitation and nutrition throughout the world. In short, saving lives is about saving lives -- not about only saving certain people's lives based on their age, gender, nationality, or economic status. And projects like the Ageless Animals research program offer both a chance to learn fascinating facts about the Earth's biodiversity, and an opportunity to uniquely (and positively) impact emerging healthcare improvements.