Default DOB(01/01/1901) is used for age calculation

What causes this issue?

The NINO for this employee is unknown and the date of birth stored in
infotype Personal Details(0002) is the default value 01.01.1901.

System Response

The NI category is determined by the employee's age which the system
calculates from the date of birth stored in the employee's Infotype 0002
record.
The NI category determined by the system may not be accurate because the
default date of birth 01.01.1901 has been used to calculate the age.

How to fix this error?

Ignore the warning if you wish to continue processing the selected NI
category you have allocated this employee.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.