。
When she was young, she liked coffee more than Japanese tea.
Literal
She [topic-は] young days [contrast-は] Japanese-tea [than-より] [also-も] coffee [subject-が] liked was.
日本茶よりもコーヒーが好きだった uses the comparative より with the emphatic も (even more than) to express a preference. 日本茶 (Japanese tea — typically green tea) serves as the comparison baseline. The past tense 好きだった and 若い頃は place this preference in her youth, suggesting her tastes may have changed since.