The query is definitely the way to go.
If I have a table with the fields id and dob in then the query needed is:
SELECT id, dob, DateDiff('yyyy',[dob],Now()) AS Age
FROM people;
Because of the way Access treats Queries almost the same as Tables you can then create Forms based on the query you have created and the Age field in the query (and the form) will get calculated as soon as you fill in the date of birth.
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Remind me to change my signature to something more interesting someday