Personally the 1 year = 1 day lifespan is a bit too short for me, I've been using a mod that adjusts my lifespan but it's not quite working for me either.
I've never been that interested in counting age of my Sims or calculating it in days. I've devised the following life-span for my gameplay. There is no relevant 1 day = X years calculation with this lifespan that I use.
*Note: I have a mod that makes shortens University to 8 days, if a Sim does not go to University 8 additional days are given to them during Adulthood to sync up their ages with Sims that did go to University. I use a mod to do this, and then I use an adjusted version of the Sim Blender mod if I need further corrections e.g. a Sim has to repeat a year.
Life-Stage | Maxis Default | Previous Mod | My New Settings |
Baby | 3 | 3 | 3 |
Toddler | 4 | 6 | 4 |
Child | 8 | 12 | 7 |
Teen | 15 | 12 | 14 |
Adult | 29 | 35-40 | 42-50* |
Elder | 9-30 | 10-30 | 9-30 |