Year known, day unknown
Enter the confirmed year without choosing an arbitrary day. An exact date narrows a query only usefully when it is correct. Filling a field for completeness adds a false constraint and can change the result.
Check component order
Different sources may write dates in different orders. Separate fields help only if the original day and month have been identified correctly. Do not choose an interpretation of an ambiguous date without checking.
Check consistency
If age is also entered, consider the time to which it refers. An age in an older record cannot automatically be treated as current. When details conflict, use only the information you can substantiate.