。。
She's not young anymore. She's at least thirty.
Literal
[topic-は] no-longer young-not. At-least 30 years-old [copula-だ].
もう若くない is the い-adjective negative 若くない ('not young') paired with もう ('any longer') — 'no longer young.' The follow-up 少なくとも ('at least') hedges the speaker's certainty about her exact age while planting a number — a backhanded way to make a point. Note 才 is the casual variant of 歳 (the standard kanji for years of age); 才 is faster to write and dominates handwritten contexts, while 歳 is preferred in formal writing. In both cases the reading is さい. Cultural note: speculating about a woman's age out loud is generally considered indelicate in Japan, which gives this sentence a slightly catty edge.