To calculate each contact’s age and show the age in the subject of relative recurring appointment in Outlook calendar, please do as follows:

1. Open the default calendar folder, and press Alt + F11 keys to open the Microsoft Visual Basic for Applications window.

2. Click Insert > Module, and then paste below VBA code into the new module window.

VBA: Calculate and show contacts’ ages in Outlook calendar

Option Explicit
Public Sub UpdateAges()
Dim xOlApp As Outlook.Application
Dim xOlFolder As Outlook.Folder
Dim xOlItems As Outlook.Items
Dim xAppointmentItem As AppointmentItem
Dim xAge As Integer
Dim xOlProp As Outlook.UserProperty
Set xOlApp = Outlook.Application
Set xOlFolder = Session.GetDefaultFolder(olFolderCalendar)
Set xOlItems = xOlFolder.Items
For Each xAppointmentItem In xOlItems
If (InStr(1, xAppointmentItem.Subject, "Birthday") Or InStr(1, xAppointmentItem.Subject, "Anniversary")) And xAppointmentItem.IsRecurring = True Then
With xAppointmentItem
If xAppointmentItem.UserProperties("Original Subject") Is Nothing Then
Set xOlProp = xAppointmentItem.UserProperties.Add("Original Subject", olText, True)
xOlProp.Value = .Subject
.Save
End If
xAge = DateDiff("yyyy", .Start, Date)
.Subject = .UserProperties("Original Subject") & " (" & xAge & " in " & Format(Date, "yyyy") & ")"
.Save
End With
End If
Next
Set xAppointmentItem = Nothing
Set xOlItems = Nothing
Set xOlFolder = Nothing
Set xOlApp = Nothing
End Sub

3. Press the F5 key or the Run button to run this VBA.

When you return to the default calendar, you will see each contact’s age is figured out and shown in the subject of the recurring birthday appointment. See screenshot:

Notes:
(1) In the subject of a certain contact’s recurring birthday appointment, this contact’s age is embraced by parentheses similar as (41 in 2017), 41 is the age, and 2017 is the current year.
(2) If you change a certain contact’s birthday in contact folder, the age will be removed from the calendar automatically.
(3) This VBA work with the default Outlook calendar only.

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