On Don Balthasar Carlos, an attractive boy of great promise but also sickly, rested the hopes of the Spanish dynasty. To strengthen the family ties between Madrid and Vienna, an early marriage was arranged with Archduchess Maria Anna, daughter of Emperor Ferdinand III. 
However, in 1646 at the age of seventeen, he died, worn out by debaucheries which had been encouraged by his preceptors. Three years later it was his father who married Archduchess Maria Anna and by her fathered the last of the Spanish Habsburgs, King Carlos II. 

Source: Leo van de Pas