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Smoking by minors is prohibited by law.
Literal
Law [by-で], minors [topic-は] smoking [subject-が] prohibited-[passive-ている]
法律で uses the instrumental/basis で — 'by means of the law, under the law'. 未成年 = minor (literally 'not-yet-of-age'). 禁じる ('to prohibit') is a formal written synonym of the more everyday 禁止する. The passive + ている marks an ongoing institutional state.