The calendar and birthdays and aging up?

As long as your sim has not reach the end of their life stage or you have them blow out their birthday cake, you should be fine. Just don't go to any birthday parties if your sim is invited, unless the sim who's doing the inviting you want them to age up.

As far as the birthdays on the calendar, the way I think of this is a sim might get a little older, but they won't jump up to the next life stage. I play on long, which means that my sim young adult live is 90 days, and my sim's calendar year is 28 days. So they will see their birthday roll around during their young adult life stage at least a little over 3 times, and this is not counting the other life stage from the time of birth to being an elder. I haven't even figured out how many birthdays they will see through their total life span.

So in short, just don't have any birthday cakes blown out. ;)

Oh that is WONDERFUL news! Thank you @LaBlue0314 <3

If your Sim has a birthday on the Calendar then they’ll age up automatically on that day. Before I got Seasons my Twin boys had just turned to children. And as soon as I played seasons their birthday was on the calendar the VERY day I started the game. Also the mom had the same birthday even though that wasn’t true. I have no idea how to fix it, but it’s really annoying. You’ll just have to age them back down when that happens.

I started a new family with toddler triplets just made in CAS, as soon as I finished building the house and unpaused the game one of the triplets aged up to a child. I'm turning aging off and will be doing it manually for all my sims.