Yes, a compound adjective like this needs a hyphen:
She was in ninth grade, but she skipped eighth-grade English and missed diagramming sentences.
Hyphenated
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."
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
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, 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.
Four-year should be hyphenated but not institution.
Year-end is hyphenated in AP Style and is an adjective.
No it doesn't require to be hyphenated.
Yes it should be hyphenated.