Bringing a sim into CAS and changing their age stage is actually not recommended unless we are talking about YA to Adult or back since those two share the same body shapes. Doing so otherwise can (not always) break the sim both internally as the different age stages up through YA have different numbers of trait slots and externally in that they can become misshapen or end up with limbs that do not fit their body properly. What we can do is replace a sim with one of another age through CAS, but that's not what most players want in most cases as the replaced sim will have no history or relationships like their predecessor did.
Other forms of death can still happen if aging is off.
What I do through mod usage is play something similar to what is called TS2 style aging as I play rotationally. When the one I designate as "the main household" is in play, everyone in town ages except for the ones I have given rotational protection. When I play other households, no one ages except the actives. Thus after one complete cycle through all of the households I play in a given world, everyone will have aged more or less evenly.
I've never played a sim whom I have intended to never age, but I have "cheated" by temporarily stalling aging on teens whom I wasn't quite finished with in their high school/teen years and adults whom I wasn't quite ready to have as elders yet. But that's not extremely different from putting them each on a daily diet of life fruit or soymilk, just without the extra bother. I think most players without mods would reach for those foods, Ambrosia, and the LTR that stalls aging to achieve a similar effect. :)