This macro runs when Outlook starts and watches for new birthday events to be added to the default calendar. When it finds one, it sets a 7.5 days (10800 minute) reminder and adds the Birthday category to the event.

To remove reminders from all day events, see How to Remove or Change the Reminder on All Day Events

Add Reminders to Birthdays

Open the VB Editor by pressing Alt+F11. Expand the Project to display ThisOutlookSession and paste the following code into the editor. Click in the Application_Startup sub and click the Run button on the toolbar. Repeat this step after making modifications to the code.

To test this macro without closing and restarting Outlook, you need to "run" the Application_Startup sub

Private WithEvents Items As Outlook.Items

Private Sub Application_Startup()
  Dim Ns As Outlook.NameSpace
  Set Ns = Application.GetNamespace("MAPI")
  Set Items = Ns.GetDefaultFolder(olFolderCalendar).Items
End Sub
Private Sub Items_ItemAdd(ByVal Item As Object)
  On Error Resume Next

If Item.Class = olAppointment And _
    Right(Item.Subject, 8) = "Birthday" Then
      With Item
       .ReminderSet = True
       .ReminderMinutesBeforeStart = 10800
       .Categories = "Birthday"
       .Save
    End With

   End If

End Sub