Indices and time spans

I have this:
     SELECT ...     FROM trade, entry     WHERE AGE( CURRENT_TIMESTAMP, posted ) <= '5 minutes'::TIMESPAN       AND
trade.entryId= entry.entryId
 

That is, fetch all trades executed in the last 5 minutes.

This query seems to run pretty slowly when trade is filled.  Putting
an index on trade ( posted ) doesn't seem to help any (the same query
plan is generated).  Any suggestions?

-itai