I was taught permanent versus temporary, and it falls apart almost immediately. Estar muerto is about as permanent as it gets, and ser joven is famously not.
Is there a rule that actually holds, or is this something you only get by exposure?
Accepted Answer
The rule that holds is norm versus change, not permanent versus temporary.
Ser describes what something is compared to its category. Estar describes the state it is in compared to how it usually is.
Es guapo says he is a good looking man. Está guapo says he looks good right now, perhaps because of what he is wearing.
That handles your counterexamples. Está muerto is a change from being alive, permanent though it is. Es joven classifies him among young people, so it takes ser.
A useful mental test: if you could say "compared to before" the sentence wants estar. If you could say "of the kind that is" it wants ser.
It is worth knowing that some adjectives simply change meaning rather than nuance, because the norm rule will not predict these and they are common.
Es listo means he is clever, está listo means he is ready. Es aburrido means he is boring, está aburrido means he is bored. Es rico, wealthy; está rico, tastes delicious.
These have to be learned as pairs. Trying to derive them from a general principle produces sentences that are grammatical and say something you did not intend, which is worse than an obvious error.
A pattern that helped me: estar covers location for things, ser covers location for events. La fiesta es en mi casa, but la casa está en la esquina.
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