Use a recordset, I think they start in Access Developer 16

You may use a calculated field for your checkbox.  Is the last contact date in the same table?  Your checkbox box could have a control source like this one:

     =IIf((Date()-[LastContactDate])) > 60, False, True)
     (Note: if [LastContactDate] could be Null, you need to modify the above accordingly.)

A calculated field is evaluated automatically for all the rows all the time, so there is no need to wait for a form to open or close.  Seems like this is what you need, because you don't do this for this one time, but for all time going forward, whenever you look at the data.

If [LastContactDate] is in another table, you may use DLookup() in the expression for the calculated field.  But DLookup() may slow things down in a calculated field because, as I said, all the rows are evaluated all the time.

If you are not storing the last contact date I would recommend doing so.  It kind of sounds like you just have  the check field.  
Check out the Contacts video and the Reminder Popup video for some ideas.

Thanks, I have 2 data tables for clients visiting our local foodbank; a ClientsTbl & a ClientVisitsTbl. The Active field is in the ClientsTbl whilst the ClientVisitsTbl carries a list of the dates that all clients have been in contact. I have made some progress with doing what i need using a recordset but am falling down in it with picking out which clients are Active vs Inactive. My definition of an Inactive client is one that hasn't been in contact within the last 60 days. I have the following:

Dim rs1 As DAO.Recordset
Dim rs2 As DAO.Recordset
    Set db = CurrentDb()
    Set rs1 = db.OpenRecordset("ClientsTbl")
    Set rs2 = db.OpenRecordset("ClientVisitsTbl")
       rs1.MoveFirst
       Do Until rs1.EOF
       If rs1!Active = -1 And Date - rs2!VisitDate > 60 Then
         rs1.Edit
         rs1!Active = 0
         rs1.Update
       End If
       rs1.MoveNext
   Loop
End Sub

If I delete the "And Date - rs2!VisitDate > 60" part of this routine then it will run & set all Active clients to Inactive , i.e. value -1 in a checkbox. However, I cannot seem to configure the "And Date - rs2!VisitDate > 60" section to pickup those clients that are still Active, i.e. they have contacted me within the last 60 days. I'm just not proficient enough with VBA I'm afraid, more used to Excel. Do I need an ELSE statement as well as IF - AND or what? Also, as I have a list of visit dates for each client how do I set up the routine to take its VisitDate calculation (for the within 60days criterion) from their most recent visit? Any help much appreciated

Try it like this:
rs2!VisitDate > Date - 60

rs2!VisitDate > Date - 60. No, that doesn't seem to do it

It doesn't know what rs2!VisitDate is, you don't set what record it is anywhere. I would just take rs2 out all togethor.

Try this, replace the ID section with your unique field if different

Date - DLOOKUP("VisitDate","ClientVisitsTbl","ID=" & rs1!ID) > 60

Many thanks Adam, following is working fine now, just had to change your DLOOKUP to DMAX. Excellent.

Dim rs1 As DAO.Recordset
Dim rs2 As DAO.Recordset

    Set db = CurrentDb()
    Set rs1 = db.OpenRecordset("ClientsTbl")
    Set rs2 = db.OpenRecordset("ClientVisitsTbl")
    
       rs1.MoveFirst
            Do Until rs1.EOF
       If rs1!Active = -1 And Date - DMax("VisitDate", "ClientVisitsTbl", "ClientID=" & rs1!ClientID) > 60 Then
      
         rs1.Edit
         rs1!Active = 0
         rs1.Update
      
       End If
       rs1.MoveNext
   Loop

End Sub