Hyphenated
It should be hyphenated when it's used as an adjective.
Yes, it should be hyphenated like so: sixteen-year-old boys.
It should be hyphenated if used as a noun clause, but not if used as an adjective
It an be either.
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.
Yes it should be hyphenated.
Four-year should be hyphenated but not institution.
The term is hyphenated as "long-lasting" (persistent, perennial, long-lived).
long-needed
Year-end is hyphenated in AP Style and is an adjective.
Yes it should be hyphenated.