Is 18 year old hyphenated

No, "18 year old" is not hyphenated when used as a noun phrase (e.g., "He is an 18 year old"). However, when used as an adjective before a noun, it should be hyphenated as "18-year-old" (e.g., "She is an 18-year-old student"). The hyphen helps clarify that the age modifies the noun.