There are a few possible reasons why this happens:
1) If the listings come to Trulia via direct feed from the broker website (company) once they are changed in the main database, it should update on Trulia. However, some agents forget to do this on their company website, even after they update the MLS listing to closed or expired.
Since most of these listings are also in the MLS, if the feed comes directly from there, then as soon as the status changes to sold/expired it will change onTrulia. (agents MUST manually change the status to closed in their MLS.... expired listings usually expire on their own in the MLS at the end of the listing contract date)
2) If a listing agent manually posts their listings to Trulia,(no direct feed) and never change the status on Trulia, that is another reason. Trulia does send email alerts that our listings are expiring etc, but I am sure some agents just ignore the alert.
Your best bet is to look at homes on real estate websites, see what's available in your price range in areas you are considering, then contact a local agent to see if they are still available, and to get more of the most recent active listings that also meet your needs The MLS is STILL the most accurate compilation of currently available homes.
Of course, many agents also have MLS searches on their websites, or you can go to the public areas of the local MLS and searchfor ACTIVE listings yourself. But why do the work, when an agent will do it for you??