If I may reply to my own question, I have found the solution.
In Settings "Mail, Conacts, Calendars", if you scroll down the the bottom, there are some Calendar settings, one of which is "Sync".
This sync setting allows one to restrict the syncing of previous events from 2 weeks ago through 6 months ago to All Events.
Changing this setting to "All Events" made no immediate difference to my iPad calendar but after I deleted my Exchange account and then recreating it, all my events back to 2001 reappeared in my iPad calendar.
I guess that this sync setting has a default in iOS 5 to that in iOS 4./