Sometimes there is only the age given. Plus there is the date when the registration was done. Doing some math one can calculate the year of the birth which must be one of two consecutive years. This information could be useful to identify individuals. So, what’s missing in the formular is a way to state e.g. „born 1921 or 1922“.
You can leave the fields for DD/MM empty and only enter YYYY. Then add a second line using the little circle-plus icon and enter the second year. Should work.
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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Thx @GHA I will go on then as you’ve proposed, even if I like the proposal of @mhitchcock more.