The first thing to remember about this topic is that we're talking about a game hobby that has spawned a science fiction series going back about two decades.
I don't say this to be snarky, but because it's important to remember that the universe of 40k and its mythos have evolved a great deal over time. Many of the authors themselves have stated that they were not necessarily completely familiar with the game's background when they wrote their stories.
So, for instance, we can reach waaay back, and read William King's "Deathwing" story. A fun story, to be sure, but where the topic of Astartes mortality is concerned, raise your hands if you think Mr. King's description of a Dark Angel close to dying of old age--and certainly combat ineffective--barely into his second century of life. Bill King isn't a bad writer; his editor, on the other hand, should have been more familiar with the subject matter. At the end of the day, it's just an example of fluff not reconciling with the intent behind the material. Such is life.
Newer fluff included references to Dante having lived for more than a millennium, and still being a feared warrior. Incidentally, I don't recall him being described as needing some sort of exoskeleton to maneuver around outside of battle. The latest description of him that I recall was in "Red Fury", where he gets around just fine and is described as merely wearing robes (at the Chapter's Fortress-Monastery).
Similarly, Logan Grimnar has been the Great Wolf of the Space Wolves for more than seven centuries. Nothing about his age indicates physical frailty.
Finally, the newest fluff pointing at Astartes longevity comes from the first few books of the Horus Heresy. "Horus Rising", in particular, makes statements about Imperial scientists whose focus is the longevity of Astartes, making claims about their subjects being effectively immortal.
There's really no reason to discount those assertions. Gerontology in large part deals with observation of the natural processes that lead to aging. There are specific, observable cycles that occur within our bodies/cells that indicate our aging process. If those processes do not occur, then the assertion of "effectively immortal" would be correct. If they do occur, but do at a rate that is not truly observable (which would make their statement technically incorrect, but not dishonest), then the effective lifespan of an Astartes would be far greater than Dante's.
Why the statement of Blood Angels being longer-lived, then?
I think the answer to that has to do with two factors:
1. The process by which a Chapter implants an Aspirant with said Gene-seed
2. Additional material used (that is, in addition to the Gene-seed)
Where the first is concerned, the original Index Astartes article on the creation of a Space Marine is rather clear: the Adeptus Astartes have largely polluted the creation process with rituals, superstition, and a varying degree of ignorance regarding what they're doing. Any number of side-effects could occur from this (and they do; consider the various Chapters who have missing implants/organs), and cosmetic effects such as a Marine not looking perpetually youthful could easily come about.
Where the second factor is concerned, consider the Blood Angels and the Space Wolves. Both have leaders who greatly exceed the ages of other Chapters' prominent figures (both Calgar and Cassius of the Ultramarines are far younger). Blood Angels "live longer", though, and Space Wolves become "Grey Hunters" not even a century into their existence.
I posit that this has to do with the additional material involved in their creation. Blood Angels don't live longer; they are enfused with the blood of Sanguinius, which grants them with a more pleasing countenance. Remember the original Blood Angels Index Astartes, which described the process by which irradiated, twisted-looking wretches from Baal Secundus became statuesque, handsome paragons of the Human ideal? Similarly, it's not like Space Wolves get older, faster; they require the Canis Helix to activate their Gene-seed. The Canis is responsible for a variety of mutations, some of which are cosmetic (leathery skin, tufts of hair sprouting through the body, gradually greying hair and elongated fangs), others of which are highly beneficial (enhanced senses).
Finally, we have the instance of the Salamander in the first book based on that Chapter (by Nick Kyme, IIRC), who survived for ten thousand years. People correctly point out that his armor was fused and his limbs had atrophied. I wonder, though, if this was due to the natural degradation of his body (aging), or more because he had spent ten millennia hardly ever moving.
To sum up: I believe that, within the context of existing in an imperfect (as are all human endeavors) body of fiction, the assertion of the Crusade-era Imperial scientists was correct. Astartes are effectively immortal, and, absent misunderstandings between authors and editors, are meant to live out lifespans that, at the very least, exceed those of Grimnar and Dante (who are both over a millennium old, and show no sign of slowing down).
In actual practice, however, the overwhelming majority of Astartes die violently long before they achieve an age even a fraction of Dante's. Similarly, their violent lives will ensure that very few maintain a pristine appearance--scars, augmetics, burnt skin, etc., will detract from this.
Finally, there will likely be some Astartes who endure such wounds over the course of their careers that they may very well become effectively crippled or reduced in mobility without actually dying. I would say such Astartes would become mentors and teachers at their Chapter's Fortress Monastery, overseeing Aspirants and such.
Cheers,
P.