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.