You don't. Instead, remove the age field completely from the table. Then,
create a query and add a computed field to that query that returns the age
(http://www.mvps.org/access/datetime/date0001.htm at "The Access Web" show
several approaches).

Use that query anywhere you would otherwise have used the table.

In general, it's considered a bad idea to store calculated fields. As fellow
MVP John Vinson likes to say "Storing derived data such as this in your
table accomplishes three things: it wastes disk space; it wastes time
(almost any calculation will be MUCH faster than a disk fetch); and most
importantly, it risks data corruption. If one of the underlying fields is
subsequently edited, you will have data in your table WHICH IS WRONG, and no
automatic way to detect that fact."

You don't do this. Calculated fields should not be stored in a table. If you
need to display age in a form or report, you calculate it as part of the
query that provides the recordset for the form or report.