Hello pascalc,
Thanks for joining and helping here! 
The proposed attitude when this situation occurs is to leave fields blank, as indicated in the help attached to these fields: „If the age is given in years in a document (e.g. 21 years), please leave the fields blank.“
However, if you do some further research and are able to determine the exact date of birth, you can add your findings in a forum thread associated to the document!
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