No, "year long" is not hyphenated when used as an adverbial phrase, such as "The project will last year long." However, when used as a compound adjective before a noun, it should be hyphenated, as in "a year-long project."
Hyphenated
Yes, "long-term" is hyphenated when used as a compound adjective before a noun, as in "long-term goals." However, when it is used as a noun or after the verb "to be," it is typically not hyphenated, as in "The effects are long term."
Yes, "five-year" is hyphenated when used as a compound adjective before a noun, such as in "five-year plan." However, when it stands alone as a noun, it is not hyphenated, as in "The plan lasts five years."
It should be hyphenated when it's used as an adjective.
Yes, it should be hyphenated like so: sixteen-year-old boys.
The term "year to date" is typically not hyphenated when used in a sentence. However, when it functions as a compound adjective before a noun, it can be hyphenated as "year-to-date." For example, you would write, "The year-to-date results are impressive," but simply "The results are year to date."
It should be hyphenated if used as a noun clause, but not if used as an adjective
It an be either.
Yes, "year after year" should be hyphenated when used as a compound adjective before a noun (e.g., year-after-year performance). However, it is not hyphenated when used as an adverbial phrase (e.g., they met year after year).
Yes, "long-term" should be hyphenated when used as an adjective before a noun, such as "long-term goals." However, when used as a noun or after a verb, it is typically not hyphenated, as in "the plan is for the long term." Always consider the context to determine the correct usage.
Yes, when used as an adjective. Year-to-year is hyphenated when used as an adjective: year-to-year comparison, year-to-year budget. Year to year is not hyphenated when it is used as a time period: We come back to this same beach year to year.
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.