。
She was beautiful when she was young.
Literal
She [topic-は] young days beautiful was.
A direct past-tense assertion with no hedging — 美しかった in plain past simply states the fact. The absence of は after 若いころ (compared to a contrastive 若いころは) makes this a neutral time setting rather than implying any contrast with the present. ころ and 頃 are interchangeable spellings of the same word; hiragana is slightly softer in tone, while the kanji 頃 leans more written. 美しい is somewhat literary compared to the everyday きれい — choosing it here lends a quietly elegant feel to the recollection.