。
She died before reaching adulthood.
Literal
She [topic-は] adulthood before [at-に] died.
成人 (せいじん) means 'adulthood' or 'coming of age' — in Japan, legally defined at age 18 (changed from 20 in 2022). 前に means 'before.' 死んだ is the plain past of 死ぬ (to die). A stark, factual sentence. The brevity itself carries weight.