This has nothing to do with whether you're a developer, but everything to do with whether you can follow a few simple instructions.

As clearly stated under the tutorial sub-heading Using the Tutorial’s Fields in Your Own Documents, implementation requires little more than copying the second field code under the Interactively Calculate A Person’s Age heading from the tutorial into your own document, then changing the 'BirthDate' references there to your 'DOB' reference, or vice versa. Of course, it would also help if the typed birth date in your document was actually in a formfield…

PS: I note that your document contains both content controls and formfields. You should not use both in the same document; they were not designed to be used that way and doing so is a known source of problems.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]