The most common and most likely answer to this crossword clue is the 6 letter word ADULTS
I've seen this crossword clue in Independent
A
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More clues leading to the result ADULTS
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The straight part
We know that "18-year-olds?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 18-year-olds? = adults.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ADULTS mean?
ADULTS noun- A fully grown human or animal.
- A person who has reached the legal age of majority.