。
She used to travel a lot when she was young.
Literal
She [topic-は] young days often traveled [used to-ものだった].
~たものだ(った) expresses nostalgic reminiscence about the past — 'used to do X.' It's more wistful than plain past tense, carrying a sense of looking back fondly on something that no longer happens. よく (often) combined with ものだった paints a picture of frequent past travel that has since stopped. The contrastive は on 若い頃 scopes the reminiscence specifically to her youth, implicitly noting that things are different now.