I found where it shows how many days are left until age up but I'm not sure how many days have passed. Oddly both sims I created as starting sims in my family show two different age days left. I found...
Hello The first post in this link thread has a list of the variable age ranges for each age stage, https://forums.thesims.com/en_US/discussion/943487/age-lengths-have-changed A patch has fixed the issue with twins not sharing the same birthday. Twins now have the same birthdays.