Twitter announced they are now displaying older tweets, when relevant, in their search results.
Twitter said that the older tweets that show up in the search results represents a "fairly small percentage of total Tweets ever sent." They use engagement metrics such as "favorites, retweets and clicks" in their algorithms "to determine which Tweets to show."
For example, I searched for [space shuttle] in Twitter and this tweet from a while back showed up at top:
Is it relevant? That is the big challenge Twitter needs to solve.
Forum discussion at WebmasterWorld.