Click on Entourage>Preferences, Calendar, and enable "Delete non-recurring
events older than xx days" to make Entourage delete older events
automatically. Alternatively, create a Saved Search (File>New>Saved Search)
to find all events specified by your criteria (e.g. "older than xx days").
You can then select all these events (Edit>Select All) and delete them.
If you need any help with that last method, post back!