DateDiff Calculate Age Incorrectly - ms-access

Question

In Microsoft Access I am using the DateDiff formula in a text box on a form to calculate a person's age. The user types the date of birth and another text box called “Age” calculates and displays the age based on that date of birth and today’s date. But for some reason the age is incorrect. Here is the formula I am using to determining age.
=DateDiff("yyyy",[txtDoB1],Date()) 'today is 2/12/2021
=DateDiff("yyyy", #5/24/1979#, #2/12/2021#) 'this has the dates manually typed in
The DateDiff formula returns 42 as the age. That is not correct. It should be 41. Why is the DateDiff formula resulting in an incorrect age? What am I doing wrong?

Answer 1

The following solution using DateDiff is from Microsoft. I have used it and it works well. The link is: https://learn.microsoft.com/en-us/office/vba/access/Concepts/Date-Time/calculate-age
Function Age(varBirthDate As Variant) As Integer
Dim varAge As Variant
If IsNull(varBirthDate) Then Age = 0: Exit Function
varAge = DateDiff("yyyy", varBirthDate, Now)
If Date < DateSerial(Year(Now), Month(varBirthDate), _
Day(varBirthDate)) Then
varAge = varAge - 1
End If
Age = CInt(varAge)
End Function

Answer 2

The DateDiff formula does not calculate the difference between years very well. If you use "yyyy" in the DateDiff formula then it only uses the year portion of the 2 dates provided in the formula to calculate the difference in years. This leads to undesirable results. In your example the DateDiff formula would take 2021 from 2/12/2021 and subtract it by 1979 from 5/24/1979. Or in other words, 2021 - 1979 = 42.
Instead try using the below formula.
=Int((Date()-[txtDoB1])/365.25) '365.25 compensates for leap years
=Int((#2/11/2021#-#5/24/1979#)/365.25)

Answer 3

DateDiff returns the difference in calendar years.
For calculating age correct for any dates, DateAdd must be used:
' Returns the difference in full years from DateOfBirth to current date,
' optionally to another date.
' Returns zero if AnotherDate is earlier than DateOfBirth.
'
' Calculates correctly for:
' leap years
' dates of 29. February
' date/time values with embedded time values
' any date/time value of data type Date
'
' DateAdd() is used for check for month end of February as it correctly
' returns Feb. 28th when adding a count of years to dates of Feb. 29th
' when the resulting year is a common year.
'
' 2015-11-24. Gustav Brock, Cactus Data ApS, CPH.
'
Public Function Age( _
ByVal DateOfBirth As Date, _
Optional ByVal AnotherDate As Variant) _
As Integer
Dim ThisDate As Date
Dim Years As Integer
If IsDate(AnotherDate) Then
ThisDate = CDate(AnotherDate)
Else
ThisDate = Date
End If
' Find difference in calendar years.
Years = DateDiff("yyyy", DateOfBirth, ThisDate)
If Years > 0 Then
' Decrease by 1 if current date is earlier than birthday of current year
' using DateDiff to ignore a time portion of DateOfBirth.
If DateDiff("d", ThisDate, DateAdd("yyyy", Years, DateOfBirth)) > 0 Then
Years = Years - 1
End If
ElseIf Years < 0 Then
Years = 0
End If
Age = Years
End Function