Try this

Sub CountBirthday()
i = 1
Count = 0
With ActiveSheet
Do While .Cells(i, 1) <> "" 'assuming datas are on column 1 (A)
If Left(.Cells(i, 1), 4) < 1960 Then 'count the number of
person born before 1960
Count = count  + 1
End If
i = i + 1
Loop
.Cells(i, 1) = Count 'display result on last row of column A
End With
End Sub