33 is old enough to have severe chronic health problems if one lives a bad lifestyle, which for Kim Jong-Un there's a lot of evidence for. We've heard of him having gout and joint problems before, and he's plainly obese. If he's really a chronic drinker on top of that, just like his father? He could easily be experiencing a health crisis, even with the care of what are presumably foreign-educated doctors (but you never know...).
Let's look at some data.
Kim Il-sung, Paternal Grandfather: Died aged 86, of myocardial infarction.
Kim Jong-suk, Paternal Grandmother: Died aged 29, cause uncertain (wartime hardship, childbirth, tuberculosis, or gunshot).
Kim Jong-il, Father: Died aged 70, of heart attack (alcoholism contributing?).
Ko Yong-hui, Mother: Died aged 52, of breast cancer.
Kim Man-il, Uncle: Died aged 3 or 4, of drowning.
Kim Kyong-hui, Aunt: Aged 71, alleged alive but in poor health, husband executed for treason.
Kim Pyong-il, Half-Uncle: Aged 63, NK ambassador to Czech Republic.
Kim Jong-chul, Older Brother: Aged 36, uninvolved in politics, plays guitar.
Kim Yo-jong: Younger Sister: Aged 30, trusted confident of Kim Jong-un.
Kim Jong-nam, Older Half-Brother: Died aged 45, assassinated with VX.
Kim Seol-song, Older Half-Sister: Aged 42, lieutenant colonel.
I couldn't find any death dates or causes for his maternal grandparents, as since they were a Japanese-Korean couple that data has been buried hard. Regardless, this is not the picture of an overly health family. His mother got breast cancer fairly young and more importantly died from it, while his father only made it to 70 and seems to have succumbed to alcohol-related damage. Kim Il-Sung did alright, but that was two generations ago and his personal doctrine of optics would have precluded any bad habits getting out of hand. All of Kim Jong-suk's alleged deaths, aside from being shot, indicate a constitution on the weaker side as well. His aunt also taking a turn for the worse around 70 is a decent correlation with Kim Jong-il's death.
His own generation only really says they don't have any young-death genes amongst them, but the half-sister lives a military lifestyle and the older brother seems to avoid the kind of involvement that would fund personal decadence. The younger sister is an interesting one for other reasons, because my research says she's actually the one who's supposed to take over for Un when he's incapacitated, as has happened before.
I think with the deaths of his parents it is totally plausible that Un could debauch himself into a downward spiral at his age.