。
She married young.
Literal
She [topic-は] young [at that stage-くして] married.
若くして (at a young age) uses the archaic/literary ~くして form — effectively 'being young' or 'while still young.' This construction is more literary than plain 若い時に (when young) and often implies that something happened notably early, sometimes with a sense of poignancy. 結婚した is the simple past of getting married.