An eighteen-year-old is ____ to vote in an election.

Enough' এর আগে adjective বসে৷
সঠিক বাক্য: An eighteen-year-old is old enough to vote in an election.

আরো উদাহরণঃ
- At the time, he wasn't old enough to vote.
- Surely no one would be foolish enough to lend him the money.
- It’s small enough to fit in your pocket.

কিন্তু Noun থাকলে সেটা 'Enough' এর পরে বসে৷
যেমন: I have not enough money to buy a car.

Source: Oxford & Longman Dictionary.