As "unit age" variable had been added in some modmodmods, where it is primarily (and possibly only) used for jotnar unipromos, i had initially suggested that mortal units shall die from old age, offended by the free-for-all immortality inherent from vanilla Civ (where warrior sent on auto-explore sometimes wandered for millenias), which had become even more striking with the "human swordsman, 348 years" on longer gamelengths. That proposition was declined for the reason of gameplay "inconvenience" - as the need to replace dying units and risks of sudden deaths of units in the key positions. I believe that main unspoken reason was that of a simple greed and fear of loss of preciouss 2k xp unit raised to these humongous proportions by countless reloads after each of its deaths. As i see now, my proposal was flawed, so a little excource into "fantasy" history and unit composition i had taken shall reveal why. First, "humongster steamroll" is not existant in any "serious" fantasy warfare writings among the authors of which i count JRRT, R. Howard and R.A. Salvatore. Dragon succeeds in sudden attack on dwarven stronghold which narrow tunnels combined with his fire breath grant him enormous advantage but falls to single archer with enchanted arrow, a huge hyena statue possessed by the chaos power first tricked into stomping allied army then disabled by being cut from its supporting mages acts as a liability instead of "game-ender" and almost invulnerable demons are being banished by drow and humans, so they amass a mortal armies in order to cause havoc more substantial than a minor distress from their unseemly visage. Second, ascendancy of tiny mortal warrior to demigod proportions is something i have not encountered in anything "serious" i read. Its either he/she/it a demigod or hero by a heritage and/or a deity's chosen, possessed/merged by/with power or is a simple mortal who by persistant hardships, trials and survival earns/wrestles some favour and empowerment, which rises him above others as, let's say, berserker or a paladin. Still aging, and still mortal by the sword. And third, as any troop is composed from the many beings who differ in the aptitude, personal experience, equipment, personalities etc, it is impossible for any troop to maintain the same (ever-increasing) efficiency level all the time. Soldiers die, retire, get wounded or crippled, break their armament, loot better one, achieve new feats of valor, learn from others, grow weak from slacking in backwater garrisons, absorb residual power from slain monstrosities, train, do many other things. A legendary human archer unit (drafted from peasantry) who with crude weapons massacres hordes of goblins, orks, troll giants, ghosts and elementals in a single battle... well... "hilarious" is the word. So the experience level of a unit is an ever-fluctuating value which never rises far beyond an average cap for its consistence type (goblins can never achieve the same heights of archery as elves or melee combat as humans and so on) and swiftly drops from removal of veterans - also because the education of novices does suffer. There was my quite detailed proposal about dividing promos into "achievable" - auto-assigned after certain combat events and "trainable" - selected after training at facilities - buried somewhere in FfH forum. And so, at last, my "age and experience" proposition: - after some bloody carnage when mortal unit had been slaughtered to some 4% strength, its exp level should drop not rise (preventing btw those situations when an attacking stack dies completely leaving a few heavily wounded survivors from the city garrison and next turn they're ultra-promoted full-staffed elite corps); - after mortal unit had reached vicinity of his composing race lifespan its experience level should steadily decline because of veteran retirement and possibly stop dropping only at maximum possible starting exp level for a new-built same type unit; - "immortality" should be differentiated from "rebirth" because by definition immortality is an inability to die from old age in safe environment (vampires are "immortal" but quite and finitely killable, while phoenix ages and dies quite often but reborn after its death - although the very "phoenix" thing looks cheesy) and "immortality" promotion should be split into itself that should prevent xp drop from aging and good old "resurrection" spell. - and yes, xp loss should lead to demotion. In psychological aspect, realisation that xp is fleeting should alleviate that "AAA my doomsday ultramegaspearman had died AAA reload/worldbuilder" syndrome.