Situation:

If the Birthday not known for the Patient, so can only the Patient age be entered.

Settings:

open the Settings Page  System\Database\Database and set the following Setting (4.6 or 5.x) 

Workflow:

(1) available "New Patient" (2) Input from Patient First and Last Name (3) Input "Age" => the "Date of birth" is allways Default by 01. 01.