To create a DOB field, go to settings then to fields
Press the plus sign next to the dropdown box to create a new field.
Select "Date of Birth" from the "Field Type" dropdown box.
Call your field "Date of Birth" or "DOB" etc and then "Save".
Add your new field to a card category so that you can have Date of Birth on your cards. To see how to add a field to a card category click here.