Complex sentence: He was educated at the Colegio de San Hermenegildo Jesuit college in Seville until the age of 15 when his parents then decided that he would become an officer in the Spanish Army, their families considering it a career suitable for a gentleman.
Split into simpler sentences: He was educated at the Colegio de San Hermenegildo Jesuit college in Seville until the age of 15. It was decided that Daoiz would be an officer in the Spanish Army, that being seen by his parents' families as a suitable career for a gentleman.