I play with aging off, but all of my Sims have a birthday that corresponds with their zodiac. I don’t have fixed dates, it is more like start of winter, middle of winter or end of winter, since I never know when I will play a Sims and I try to have them have their birthday when I play them.
I don’t age up the Sims to the next age stage on their birthday though, they only age one day.
So I age them up on their first birthday, I make a note in the biography of the toddler then, because since babies lifespans can’t be set lower than two days I skip one birthday for the toddlers to make the age right again.
Toddlers age to children on their 4th birthday.
Baby: 0-1 years old
Toddler: 1-4 years old
Children: 4-12 years old
Teenagers: 12-18 years old