A frequent request is the ability to display a person's age on their birthday event in Outlook's calendar. We used to say "you can't" and pointed users to our custom form that displays a contact's current age.
We've since learned of a way to use custom fields to populate the subject field – one use of this is to add the age to the subject field of an appointment. Uses for this are not limited to displaying ages – the person who showed me the trick used it for the subject field of a task form.
It's not perfect and the contact form method might be a better solution for most people, in part because the age is always up-to-date. The age doesn't automatically update in the appointment form – you need to touch the subject field (adding a space to the subject will do it) to update the age.
Instructions and a sample custom form are at How to create a custom appointment form that displays a person's current age in Microsoft Outlook calendar
Published May 23, 2011. Last updated on September 10, 2011.