Hi @halimali1980-
I figured it out!
I was so used to Sims aging up at 6pm (I skipped Sims 3 (didn't care for story progression and whatnot), so I'm sure if that was still the case), and it dawned on me that the Sims age at the time you create them. Second, the game makes it so you can't keep going back to a household after your sim's "aging time." Let me explain:
Say "Lucy" is your sim and she was created at 1:00p. Every time at 1:00p she ages a day. You continue to play "Lucy" till 10:00a. Then, say you switch to "Jason", and play him from say 10:00a to 3:00p. Then at 3:00p you switch back to Lucy. Lucy will have aged 1 sim day, because her 1:00p age time had passed as well considering the fact that you played her that day. This prevents sims from never aging at all, as you could, theoretically keep visiting her after her "age time" and having her not age. That is why this feature makes sense.
I hope that made sense. This was seriously baffling me for an entire day, and thought this would be valuable information to share with the community.