Indeed, the young prince was considered to be so sickly that no effort was made to educate him. The reason? In case the stress of learning was too much for his weak constitution. He was also given license to be completely lazy and rebellious. So much so, in fact, that at times he was notably dirty from not cleaning himself.
Despite this catalogue of failings, however, Charles was indeed crowned King of Spain when his father Phillip died in September 1665. At this point, Charles was still weeks away from his fourth birthday, so his mother – and cousin – Mariana was appointed regent to rule in his place.