By default, if Cached Exchange Mode is enabled, you will only be able to view messages from the last 12 months. This keeps the Outlook file smaller which allows you to browse your email faster.
To temporarily view messages older than 12 months you can scroll to the end of an email list in a folder and click 'Click here to view more on Microsoft Exchange' or 'More'.
To change your view permanently you can change the setting in account settings.
Click Account Settings on the File tab, and then click Account Settings.
In the Account Settings box, double-click your Microsoft Exchange account on the Email tab.
In the Change Account dialog box, drag the Mail to keep offline: slider to the desired number of months.
Click OK when advised you have to restart Outlook to complete the configuration change.
For more information see http://support.microsoft.com/kb/2733062