How to validate DOB for certain contact relationship types

Summary:

We require date of birth to be entered for certain relationship types (those who can be dependents in benefits.) I was able to enable the validation when ADDING a contact, but I can't figure out how to apply the same validation to UPDATING a contact.

Content (please ensure you mask any confidential information):

I pasted the code I am using where contacts can be added in Benefits: before you enroll, report a life event, and self-service enrollment. Can an additional validation be added to prevent an employee from removing the date of birth after it has been added and saved? Every place you can ADD a contact, you can also EDIT the contact, so I would like to have a similar validation to prevent the date from being removed. (Yes, it has happened! Only once so far, but if I can prevent it, I would like to.)