Slann are biologically immortal and their bodies are largely extraneous to their magical dominance and spiritual magnitude, like as proven by Relic Priests, even when killed, they don't fully die.
Saurus are biologically immortal and noted to literally grow eternally stronger and tougher as time goes on, scales and muscles growing denser and thicker and gaining spikes and so on, even as they get scarred up all to hell. Also grow more experience and capability in tactics and strategy quite effectively. Not dumb, but super singularly focused on military matters, like, it's all they ever think about, threats, dangers, combat, war, siege, defensive action, etc. and little else, ever. No real contemplation on stars, plans of the Old Ones, or anything like that, near zero capacity for even conceptual understanding of leisure save as something other beings do.
Kroxigor are biologically immortal and incredibly dull in intelligence most of the time, built very well to follow orders, and little else, dropping weapons and tools the moment a task is done to move into the next. Used more for construction, repair, etc. and combat as secondary occasional necessity as decided by others. A lot of the time they act out an eternal task list in order, with orders added or subtracted by other Lizardmen. But little sign of constant physical improvement like saurus, but they do get festooned with more and more talismans, trinets, etc. as they continue living, and some are just straight blessed, like Nakai.
Skinks are numerous, dextrous, far more mentally open and flexible compared to saurus and kroxigor, from being fishermen, warriors, trackers, fliers, priests, attendants, interpreters, organizers, etc.
But they are not biologically immortal in regular canon as near as I can tell.
An exception exists in Oxyotyl, who got trapped in the Realm of Chaos during the Great Catastrophe, and popped out thousands of regular causality years later, and pops in out of sight and existence, and went through such hell and mind bending trials that Slann straight up will not touch his brain lest they themselves be driven mad and be corrupted. Besides, he doesn't even pop out until 2500s, so Realm of Chaos aside, for all we know he's just a regular biological age skink for all cumulative real space time.
Another exception only sort of exists in Tehenahuin, but in that whenever he dies or goes away, inevitably another red crested skink shows up claiming to be the Prophet of Sotek, and is recognized as such by the worshippers of Sotek.
It would seem impossible for a short-lived Skink to attain this length of years, but it is whispered that as a snake sheds its skin and is renewed, so too is Sotek's Chosen. Since those days, at times of great import a single red-crested Skink Priest has emerged from the deep jungle, claiming the tide o f Tehenhauin, the once and future Prophet of Sotek.
Like, either original T died more'n a thousand years ago, or he is a specifically insanely long lived skink specifically because he is the chosen Prophet of Sotek.
On the other hand, the incredibly super talented oracle and foresight fellow EVA mentioned, so respected even Slann think he's all right, worthy of Slann-grade Temple Guardian Saurus, is ancient at around 100 years old in 2520s, carried around on a palanquin because of how crazy old he is and frail because of OOOOOOOLD.
Possibly all of which is because of the abundance and regular spawning of skinks. Zero Slann are spawned, kroxigor are spawned in accompaniment to skink spawnings, and saurus are spawned, and skinks are spawned in red crested, oracle, priest, regular, or chameleon varieties all in great number. So....yeah.
