You could say "Few eighteen-year-olds pass the exam," but since you used "only" you have to use the indefinite article ("a").
We tend to use "plenty" with nouns (especially noncount nouns). Examples: plenty of money, plenty of honey, plenty of water, plenty of soda.
It is common to use "much" with adjectives of comparison. Examples: much warmer, much harder, much colder, much easier.
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