One wouldn't expect to see the formula - it's in a field code in the cell concerned. To see it, unprotect the document and press Alt-F9. In the cell concerned, you should see something like:
{QUOTE{IF{=Birthdate \@ YYYYMMDD}= 0 "" "{Set Years{=INT(({DATE \@ YYYYMMDD}-{REF Birthdate \@ YYYYMMDD})/10000)}}{Years} Year{IF{Years}= 1 "" s} }"}
It's a pretty simple formula, really.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]