I have noticed this weirdness from Day 1 with the Base Game, back when it came out in June of last year. I am not crazy about it, but, oh well. However, I am noticing a little bit of a trend, but I haven't really tested it sufficiently enough to know if this is really what is determing my Sims' initial lifespans or just a coincidence. However, I have noticed that the, for lack of a better word, fatter, my sims are when created in CAS and the less muscular they are, the more days they seem to have chopped off of them on Day 1. When I've made muscular, slim (not skinny) men, in the normal lifespan, they usually have started at 20 to 21 days until becoming an Adult. When I have made plumper and less muscular men and women, they have started out with 18 to 19 days until becoming an Adult. I made one particular female Sim who was quite obese (though not all the way to the right side of the slider) with virtually no muscle tone and she stated out at 16 days before becoming an Adult. So I think the game is making it's own lifespan analysis, at least right after creating a Sim based on how "healthy" they seem right out of CAS. Like I said, I'm not too crazy about it, but if I am onto something here, I can sort of understand it.
~Layla~