。
She is five years old.
Literal
She [topic-は] 5-years-old is.
歳 (さい) is the standard counter for years of age, with the irregular reading 二十歳 (はたち, '20 years old') as a notable exception. The simplified form 才 is widely accepted in casual writing and on signage. The polite copula です anchors the sentence in everyday register. Saying '5 years old' alone — without なる ('become') — describes the current static state, distinct from the ~になる pattern that marks change-of-state ('became X years old').