You should change your text formfield's internal bookmark name to 'BirthDate' and use a field constructed as:
Code:
{QUOTE
{SET sy{BirthDate \@ yyyy}}
{SET sm{BirthDate \@ M}}
{SET sd{BirthDate \@ d}}
{SET ey{Date \@ yyyy}}
{SET em{Date \@ M}}
{SET ed{Date \@ d}}
{Set Years{=ey-sy-(em<sm)-(em=sm)*(ed<sd)}}
"{Years} Year{IF{Years}= 1 "" s}"}
Do make sure the 'BirthDate' formfield has the 'calculate on exit' property checked.
Note: The field brace pairs (i.e. '{ }') for the above example are all created in the document itself, via Ctrl-F9 (Cmd-F9 on a Mac); you can't simply type them or copy & paste them from this message. Nor is it practical to add them via any of the standard Word dialogues. The spaces represented in the field construction are all required. Although I've used linebreaks in the field code, they're no needed - they just add clarity.
To see how to do just about everything you might want to do with dates in Word, check out my
Microsoft Word Date Calculation Tutorial (which is where the above field code comes from):
https://www.msofficeforums.com/word/...-tutorial.html