I have looked and looked and can't seem to find any reference to what's happening in my game. I know that the physical trait extends life in the Elder stage. I have also found reference that leads me to believe that other factors change the rate of aging slightly. Some things I've seen mentioned (I couldn't tell you where as I've been all over the Internet trying to track this down) are completing whims and aspirations causing delayed aging and lots of misfortunes speeding it up. I couldn't find anything definitive on these items, however, only suggestions.
So, what's happening in my game is this ... I created a couple in CAS, put them in a home, and started playing. When they reached their Adult stage, they were a day apart with him aging one day before her. I figured she was a little further in her aspiration, so maybe that really did impact it (they were almost the same in whims and misfortunes). I continue playing and didn't think much about the age until I got notice that he was going to become an Elder soon. He had 3 days until aging, and she had 15 days!
I checked, and their stats are nearly the same, with him actually having about a dozen more whims completed than her. However, she just finished her aspiration, and he's got one step left. I waited a couple of days, and he aged up as expected, except now she has 16 days until aging. So now, her age isn't just slower than his, it seems to be going backward.
I've not used any rewards, potions, mods, cheats, or anything I'm aware of that would cause her to go backward or stall in age. Has anyone else had this happen? Does anyone know if completing aspirations and whims or experiencing misfortunes actually impacts the aging process? I've noticed other couples aging differently in the game, but nothing this drastic (as couple of days usually by the time they are elders).