There appears to be a rough progression to the depiction of the First Ones appearances, which might tell a little bit about their behavior and life cycle.
The very First One is Lorien, who is only seen in his true form very briefly. He appears a huge sphere of energy from which pseudopods extend. Lorien is thus very much like a titanic single-celled organism-- perhaps an amoeba. The choice of making Lorien a bit like an amoeba is quite insightful, since the amoeba divides by equal fission and effectively makes every organism billions of years old. There is no way to determine which of any two 'child' amoebas is the original from each split. Trace that back infinite generations and you have an incredibly old _individual_ micro-organism.
The Vorlons in their true form also appear as gigantic energy-based versions of micro-organisms. They appear as giant hydras; tubular organisms with many flailing tentacles. There is a vague evolutionary progression, in terms of complexity and age of origination, between amoebas and hydras. I do recall that hydras can reproduce by budding; Vorlons can do this in the sense of breaking off portions of their consciousness and putting it inside other beings. Budding could also explain the intense and complicated relationship betweek the ancient and wise Kosh and the more tempermental Ulkesh; the latter may have been grown from the former's flesh. It could also explain why Ulkesh insisted upon being referred to as Kosh following his 'parent's' demise. Their personalities and age may have differed, but their identities may have at one point been one single being.
Shadows also appear as 'primitive' lifeforms; they look like a blend between spiders and perhaps crabs or lobsters. This breaks the pattern in part, because at one point during the series, it is mentioned that the Shadows were older than the Vorlons. I guess one can wriggle around that, by considering that the Shadows may have been slightly less evolved than the Vorlons-- it might explain why they were not glowing entities of light like Lorien and the Vorlons. In any case, the Shadows were still beyond being composed of matter-- their ability to vanish from sight may well have been an indication of their partial immaterial nature.
(Tangent: I suspect that if the Shadows had been explored more thoroughly, it would have been revealed that they reproduced by impanting larvae in other beings, much like the xenomorphs of the Alien movies and the Magog of the Andromeda series. This would explain their affection for parasites like Keepers, that burrow into the flesh of a host. Not to mention their indifference to the suffering of individual beings.)
To me, this suggests that JMS saw that the earliest of the First Ones were immortal because though they were intellectually quite advanced, their bodies were relatively uncomplicated. Much like a multi-function gadget, generally the more complicated something is the faster it is probably going to break. The same may apply to living things in the B5 universe.
For this reason, the Vorlons probably lived longer than Shadows simply because their bodies were simple and less prone to fatal injury. It took _many_ Shadows to destroy Kosh, and it did not seem a quick death either.
The Shadows may well have been older than the Vorlons, but few if any had survived from the days when Lorien had charged them with the duty of shepherding the younger races. As a result, the older Shadows which might well have understood their role best were gone, and their impatient and cruel descendants instead obliterated whole worlds quite casually. The ancient Shadow leaders may well have had an enlightened attitude towards their philosophy of Chaos, but the brats only had a taste for blood and a dogmatic love for strife. The ancient Shadows in fact may have had more understanding than the ancient Vorlons, because they held the lair of Lorien with a religious regard that seems totally absent among the Vorlons.
The Vorlons by contrast were apparently longer-lived than the Shadows, because they still had a few patriarchs like Kosh Naranek who did not live by the 'letter of the law' and could remember when Lorien walked among them, perhaps as a teacher. Kosh understood that the role of the Vorlons was to use Order to foster growth among the younger races-- not prove their ideology through genocide, as eventually occured.
Sure, I'm analyzing it to death... but this appears to be an asylum for people who share some of my affliction...
