Birthdays with rotational play and calendar

I think the only way a sim would age up would be if they blew out their birthday cake. I think (and I hope I'm not wrong) but the birthday doesn't have anything to do with them aging up to the next level, but I think it just has to do with that they are a year older, not a whole life stage older. I mean by that is I have aging set for long, and my seasons for 4 weeks long. A year for my sims is 28 days, and my young adult sims live for 90 days (not counting the other life stages). So you shouldn't have to worry about them aging up, just don't have your sims go to any birthday parties, to avoid any unnecessary aging up, from the other sims as well.