Since it's not included on the elements' list in forms, here's a trick how to add it to your forms to fill your contact's 'Date of Birth' field inside NexLvL database.

Capture Date of Birth?

Click on Database

Click on Forms

Click on Advanced

Drag and Drop Date / Time

Label

Change it to Date of Birth instead.

Click on Field

Click Save when done. The system will automatically recognise it as 'Date of Birth' when the form is filled out, thus, same will reflect on the database input.