I am then trying to setup a new "age" field in the original table.
You should not store such calculated values in any table. In a form textbox, show age as = Date - DOB (or whatever you call it) assuming that the value in the table field is the entire date and not just a portion of it. How you format the control depends on what you want to see. If only whole numbers and you don't care about rounding, then Integer values should work. You can try a ready made format on the property sheet for the textbox.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.