how to calculate age for access

There are a variety of ways to achieve this, here are 2

Age: DateDiff("yyyy",[Date of Birth], Date()) + (Date() < DateSerial(Year(Date()), Month([Date of Birth]), Day([Date of Birth])))

Another approach is to simply use a reusable VBA function like:

http://allenbrowne.com/func-08.html

Or

http://www.devhut.net/2010/06/22/ms-access-calculate-the-age/

and call it as required.

To get it right for any combination of dates, DateAdd must be used, like here:

' 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