Annnnnnnnywayyyyy...
So, I went ahead and found out all the lifespan extension stuff. (Afik)
Races start out with 72 years, you get +10 from Vitality Boosters, +10 from each of the lifespan extension policies (for +20 total), +20 from one of the first things in the harmony tree, and +5 years from each tier of the repeatable life extension technology. The accession paths all have their own lifespan boost, Synths are immortal of course, and the genetic dudes can probably pick up a LOT of lifespan, but it's probably not worthwhile, psi have a fairly common event for +40 lifespan (that's what many of my dudes in the screen shot have). There's also the personal trait that gives 20 years of course.
So, I'm thinking realistically, to get to the end of the tech tree with a low leader lifespan, you could be egalitarians who are on a super conquering spree so your unity tanks and you never bother to pick up harmony or ascend. That'd leave you with a 82 year lifespan, or probably around 40-45ish years of service from your dudes. At least until you hit the repeatable techs. Although that's still not 75 years (afaik it's impossible to get to the repeatable tech with such a low min age, so I'm guessing it was either: Not old age, some negative trait, or some other version of the game) it's still pretty low and you would have to swap out leaders much more often. Of course if you take over the whole galaxy like that the game's probably going to end pretty quickly, so give and take on that front.