Two of Philip and Mariana’s children survived into adulthood: Margaret Teresa and her younger brother Charles. Despite being ten years younger than his sister, Charles was the heir to the throne because he was male. As such, he was given the title “Prince of Asturias,” which traditionally went to whomever was due to inherit the Spanish crown.
Of course, the fact that Mariana was Phillip’s niece meant that her son Charles was in fact her first cousin, as well as his father’s great nephew. Such are the complexities of having children by close relatives. And this interbreeding may well have been a factor in the poor health, both mental and physical, that plagued Charles from infancy.
Charles was four years old before he acquired the power of speech, and he was eight before he could walk. Consequently, he was treated as though he were a toddler until the age of ten. He also suffered from a quite severe deformation of the jaw, a characteristic abnormality among those of the Habsburg dynasty. This disability meant he had great difficulty with both eating and speaking.