checked your settings /Mail Contacts Calendars/ Calendar Sync settings?
you can set that to a variety of time periods (2 weeks 1 month 3 months 6 months and All)
on one of our iphone5 that was set to 1 month I had to
quit the Calendar app
go into the Mail Contacts Calendars settings choose the iCloud account, turn the Calendars off for the iCloud account, when prompted chose 'delete from My Phone', then turned Calendars back on for the iCloud account.
This seemed to refresh the Calendar entries and sync the older ones
Bear in mind that even if you can see them when manually scrolling thru calendar entries, if you search for them in a spotlight search it wont return any results for ones that are more than 1 year prior to current date set on your phone.