This is a simple plugin which will show a warning at the top of any old topics. You can define an "old" topic based on a specific date cutoff, or by a number of days old (i.e. to show a message at the top of any topics that are more than 365 days old). You can specify the date for date-based cutoffs, or the number of days for the days-old cutoff.
The message is displayed at the top and bottom of every page of the topic. This simple utility can help prevent super-old topics from being bumped when users reach them from search engines.