Even when you aren't concentrating on a family, they are busy taking care of their needs and aging. This new feature is called Story Progression. Control over everything is taken out of your hands and given to Sims not being actively played. Sims' friends will now age with them. No longer with a Sims best friend be left behind at the child stage, while your favorite family who is played a lot more goes away to college.
Story progression ages Sims, allows them to get married, and have babies. When you go back to play the single Sim you created, you could find them married with a coupe of kids. How cool is that?
"The Sims 3" was designed with the thought you play one family per a neighborhood. EA was thinking and did not leave this option set in stone. Story progression can be disabled and free will can be lowered or turned off.
To change free will and story progression settings, press 'F5' and choose Options. There is a slider for free will and a checkbox to enable or disable story progression.