。
She was beautiful when she was young.
Literal
She [topic-は] young days [contrast-は] beauty was.
A straightforward past statement without the inferential に違いない — the speaker states it as known fact rather than deduction. 美人だった (was a beautiful woman) in the past tense with 若い頃は implies she was beautiful then but not necessarily now. The contrastive は on 若い頃 subtly underlines this temporal contrast.