。
She'll turn 17 this coming February.
Literal
She [topic-は] this-coming February [at-で] 17 years-old becomes.
The frame [time]+で+[age]+になる is the standard way to say 'turns X at time Y' — で marks the punctual moment at which the new state takes hold. Compare 'もう５歳になる' ('will be 5 already') with the temporal で: '今度の２月で' fixes the moment precisely. 歳 ('years of age') is the standard age counter; the variant 才 is its simplified form, common in informal writing and on signs. Becoming a particular age in Japanese is framed as 'becoming,' not 'being' — the change-of-state verb なる underscores the transition.