。
She was very beautiful when she was young.
Literal
She [topic-は] young time very beautiful was.
たいへん (very, greatly) intensifies 美しかった. 若いとき (when young) uses とき to mark the time period. This is a direct factual statement about the past — no inference markers like に違いない, just a plain past assertion. たいへん is slightly more formal or emphatic than とても.