The correct answer is 'you are old enough to'. Key Points 'Enough' can be used both as an adjective and an adverb. If used as an adverb, it is placed after the adjective it modifies and if used as an adjective, it is used before the noun it modifies. Example, She is intelligent enough to clear this exam in her first attempt. I have enough time to study. Here, 'old' is an adjective. We need an adverb to modify it, so should be placed after 'old'. According to the rule and examples that are given above, 'you are old enough to' will be used in the underlined part of the sentence. Correct Sentence: If you are twenty one you are old enough to drive a car. Additional Information Elder and older but mean "more advanced in age." But while "older" can describe things as well as people, "elder" is reserved for people only. "Elder" can also be used as a noun, as in "respect your elders," whereas "older" cannot.