。
Muiriel turned 20.
Literal
Muiriel [topic-は] 20 years-old [target-に] became [polite-past-ました].
~になる ('become X') with に marking the resulting state is the standard pattern for expressing change. 20歳 is a milestone in Japan — the age of legal adulthood until 2022 and the age of seijin shiki (coming-of-age ceremony). The action is described as already-completed (なりました) rather than as a present state.