LEGISLATION
Spanish law does not provide for the right to deny access to a restaurant/hotel based on age.
Prohibiting children from entering a restaurant/hotel is as legal as stating that people of a certain ethnicity or nationality cannot enter.
Law 7/2013, from November 26, de régimen jurídico de instalación, acceso y ejercicio de actividades en las Islas Baleares (on the legal regime for installation, access and exercise of activities in the Balearic Islands): "The staff of public entertainment and recreational activities and public establishments will exercise the right of admission respecting the dignity of people and their fundamental rights and may not imply any type of discrimination, both with respect to access and permanence, as well as to the use and enjoyment of the services they provide."
https://www.boe.es/buscar/doc.php?id=BOE-A-2014-655. Article 19. Right of admission.
SOURCES AND RESOURCES
Este local no admite menores, ¿el auge de la 'niñofobia'? (This place does not allow minors, the rise of 'childphobia'?) by Amaya García and Ana del Barrio,
Las mentiras de los hoteles solo para adultos (The lies of adults-only hotels) by Pablo Gracia,
FACUA - Consumidores en Acción is a Spanish NGO dedicated to defending consumer rights, Phone.: (+34) 688 954 954/ 669 200 100.