@plau Make sure you have the most recent version of MCCC (2022.5.0) and according to the changelog:
Fixed aging issues caused by the HSY update. EA changed default age spans for several ages (with no notices in the change log on their website , I might add!) so that caused differences between the number of days the Sims have lived and the life spans which caused some Sims to be immediately ready to age-up on starting the game. Teen was changed from 13 to 21. Adult was changed from 24 to 33. Elder was changed from 10 to 14. Workarounds should no longer be necessary for games with different aging speeds now either. From my testing, works fine on Short, Long or Normal (although there is still really not too much reason to play on anything but Normal just using whatever age-spans you want for the Sims in that…)
Try changing your lifespan to normal and then input however many days you want for each life stage and see if that helps with your issue. This is what I did and I haven't had any issues like you're describing. I hope this helps. :smile: