"The boy is not an adult."
Translation:El niño no es un adulto.
Why "es" and not "esta"? Presumably at some point the boy will be an adult so the state would appear to be temporary
When they say "estar" is used for temporary conditions, they mean conditions that are readily changeable and can change back and forth regularly. Like clean/dirty, wet/dry, well/sick, happy/sad, tired/rested.
Use "ser" to describe characteristics that could change over time, but that are not expected to change back and forth regularly: El chico es bajo; Mi casa es verde; La chica es joven; Tu coche es nuevo.
that's just how it is... just something you gotta memorize. maybe because it won't change for a while or quickly