The Sims 2 Guide and FAQ

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5.5 Friends between age groups
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Sims can become friends with any Sim of any age, however there are some things
that can not be done between age groups. The good thing about this is that it
does not matter what age the Sim is when you first meet them. This will only
effect the way the relationship progresses.

I have found getting Adults Sims to make friends with younger Sim can be a good
tactic as this will prevent any accidental jealousy problems that can occur
when having relationships with other Adult Sim.

Also having a good friendship between age groups with a family will help is you
need to change the personality of the younger Sim. See section 1.3.2 for more
information on this.

There are no limits to what friends you can have and over what age groups.