You can either start out completely single or enter the world of The Sims 3 with a family and friends. After creating a Sim, click on the Add a Sim button to start building a new Sim. You can work in any of the age groups, from toddler to elder to build anything from a massive clan to a young twosome just starting out in life.
Family
The quickest way to grow your Sim family is to have a child. Once you have created two Sims of adult age, one male and one female, you can pair them up and automatically generate a child. Basic laws of genetics kick in and some physical attributes from a parent Sim are passed along to the child. For example, if one parent has blond hair, there is a good chance the child will also have blond hair.
To create a child from two adult Sims, choose the “Make a Child” option. This brings up a window with your available adult Sims. After selecting two adult Sims, the child appears below. You can adjust the gender of the child and the age.
Actually, you can do some pretty wild stuff here. You can create a child but then age the child into an elder right away and then place it in the household. The elder is not the couple’s child anymore, but it’s an interesting way to play with genetics.
Once you have created a second Sim—or a variety of Sims—you can start assigning their relationships. The most basic relationship you can assign between two Sims is “housemates.” Any of the Sims of any age can be housemates.
If you do not assign this relationship between members of a household unit, it is ascribed by default. You can make two adult or young adult Sims spouses (different genders is not a requirement for spouses). Younger Sims like teens or children can be assigned as siblings.
Drag-and-drop Sims on top of each other in the family tree to assign relationships, such as spouses, siblings, or housemates.
Aging
Sims are not static creatures. They age just like you and I do, but certainly at an accelerated pace. As each day inside the game passes, your Sims age a little bit, inching ever closer to the next phase in life. Each life phase lasts for a specific number of days.
Toddler:
7 days
Child:
7 days
Teen:
14 days
Young Adult:
21 days
Adult:
21 days
Elder:
16 days minimum
When a toddler ages into a child, the Sim earns an extra trait. This continues with each phase until the young adult phase, which has five traits—the maximum for a Sim. Traits are carried over from the young adult through the elder unless a specific trait is incompatible with the age.
The final phase in life, the elder phase, culminates with the inevitable: death. However, as long as there are children in a household of Sims, the household will continue to thrive.