Create a macro on a date field

mcgannmary 

First, that calculation is going to be inaccurate half the time. People age on their birthdate, not on the last day of a year. So on any given day PRIOR to that birthdate during the year they will be 17 and on any given day AFTER that birthedate they will be 18, and so on, year by year.

There are a number of ways to calculate current age accurately as of the current date. Here's one. 

Function AgeOnLastBirthdate(ByVal Bdate As Date) As String

Dim intPreBirthdayYear As Integer

intPreBirthdayYear = Date < DateSerial(Year(Date), Month(Bdate), Day(Bdate))
AgeOnLastBirthdate = DateDiff("yyyy", Bdate, Date) + intPreBirthdayYear & " yrs"

End Function

Second, this is going to be a lot easier in VBA than a macro. Macros are simple, yes, but kind of too simple.

Use the Before Update event of the control on the form.

Private Sub DateofVisit_BeforeUpdate(Cancel As Integer)

If CurrentAge(DateofBirth].Text) < 18 Then

    Cancel = True

    MsgBox Prompt:="Patient must be at least 18", Buttons:=vbOkOnly, Title:="Too Young"

    Me.DateofVisit.Undo

End if

End Sub 

You have to use the .Text property of the control instead of the .Value property because the control is not yet saved.