My sims were half way through young adult when I checked their ages tonight. Three sims that I had just made the moment I loaded the game up tonight. I made these sims and entered build mode the second the lot loaded up, so I don't know if they started that way, or if build mode also causes the issue.
I then tested entering CAS. All three of the sims were at max age. Disabling aging obviously stops it, though. Although, I didn't check to see if non-played sims were also aging. None of the passersby seemed to age, but I didn't check to make sure.
Edit, tried the tutorial.
The sim I created for the tutorial also aged up when I entered CAS. I tested it once I played through the tutorial.
As for being half-way through life stage, that happened in the tutorial as well, but that looks like a conflict between modded life stage lengths. Sims start at the base age. So 24 days until adult from young adult. Seems to completely ignore the mod.
And yes, I did try without mods. The only thing that changed was that my sims weren't halfway through their life stage.