。
She celebrated her 15th birthday yesterday.
Literal
She [topic-は] yesterday 15-years-old [genitive-の] birthday [object-を] celebrated.
祝う ('to celebrate, congratulate') takes its object directly with を — 誕生日を祝う (celebrate a birthday), 結婚を祝う (celebrate a marriage), 合格を祝う (celebrate passing an exam). The full-width digits １５ are typographically 'wider' Latin characters that Japanese text uses interchangeably with half-width, especially in formal or older typography. The counter 歳 ('years of age') reads さい normally — and reading it on this number gives じゅうごさい. Note the contrast in cultural weight: 15 is not a major Western coming-of-age year, but in Japan 七五三 marks 7, 5, and 3 for younger children, while 成人式 ('coming-of-age ceremony') marks adulthood at 20 (now 18 since 2022).