Legal age for marriage is 18 years for both men and women.
Bride and Groom may not be related.

Usually one of you must be resident for two years before the wedding or own property in Spain. Alternatively, if one of you is Catholic then you do not have to be resident and you can have a Catholic ceremony.

You must bring the papers mentioned below and fill in an application form to marry at the Civil Registry where the marriage will take place.
Your intent to marry notice must be posted for fifteen days before you can marry and must be sworn by the applicants.

Original birth certificates certified and translated by an approved translator and notarised with an apostille.
Passports.
Certificate of No Impediment to marry (usually issued after the intent to marry notice has been posted for fifteen days).
Certificate of Residence.
If you have been divorced then the final divorce papers must be shown.
If you are widowed then the death certificate of your previous spouse and your former marriage certificate must be shown.
If your name has been changed by deed poll then legal proof must be shown, stamped and dated by a solicitor (including if you are divorced and have reverted to your maiden name).
Please note: the documentation required varies from state to state in Spain and some, if not all documents will need to be translated and notarised with an apostille seal.