i'm not sure if it's already been explained outright in the manhwa but basically
the seo family is cursed where their descendants are in danger until they reach 20 years old, but if they manage to survive past 20, they won't be under the curse anymore and can live life normally
it's not a "every descendant in the seo family will die before 20" curse, it's a "every descendant in the seo family is in danger before they reach 20, and most don't make it" curse
seungjoo's parents are both still alive
also slight novel spoilers below
seungjoo's older sibilings didn't make it to 20. his oldest sibling (aka the one muheun was 'paired' with as a guardian like how muryeoung is paired with seungjoo) was close, but fell to the curse in the end
OOOHH I SEE. Now it makes sense!! I was under the impression that "every descendant will die before reaching 20" and seungjoo is one special case LOL. ((It even made me think of ubuyashiki clan of kimetsu no yaiba who marries and have kids at early age :'))) thanks for ur explanation!! (づ￣ ³￣)づ
One thing I've been wondering is how the MC's family (seo family) continues to go on? It said that they are known not to reach 20 years old (with MC as an exception)... im assuming his parents are ded (but were able to have 3 children with long age gaps?). It seems that he doesn't have any niece/nephews from his older sibs either... Tho maybe they have relatives... （￣へ￣）(yes im overthinking these things lol dont judge me im just curious xD)