Can I edit an existing demographic field?

You can edit any demographic field that you have created. Preset demographic fields such as "First Name" are not editable.

Can I delete a demographic field?

No, you cannot delete a demographic field. You can disable it so it's not available in your forms and options for that mailing list.

Is a demographic field available for all my mailing lists or only the mailing list that I created it in?

Your created demographics will be available to all mailing lists within a single account. To activate the demographic in any mailing list, you have to go to each mailing list to enable it directly.  

Do demographic fields need to be enabled for me to use them in forms such as the Opt-In Form?

Yes. Only demographic fields that you have set to enabled will show up as options in your Opt-In Form or other forms.