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Minors are forbidden to smoke.
Literal
Minors [topic-は] smoking [object-を] are-prohibited.
禁じる ('to prohibit') in its passive form 禁じられる — 'is prohibited.' The ~ている adds the ongoing-state sense: not 'has just been prohibited' but 'stands prohibited.' 未成年者 combines 未 (not yet) + 成年 (adulthood) + 者 (person) — 'one who has not yet reached adulthood.'