20 year old or 20-year-old?

[Raptor]

One for the writers:

Many compounds that are hyphenated when used attributively (adjectives before the nouns they qualify: a light-blue handbag, a 34-year-old woman) or substantively (as a noun: she is a 34-year-old), are usually not hyphenated when used predicatively (descriptive phrase separated from the noun: the handbag was light blue, the woman is 34 years old). Where there would otherwise be a loss of clarity, a hyphen may optionally be used in the predicative form as well (hand-fed turkeys, the turkeys were hand-fed). Awkward attributive hyphenation can sometimes be avoided with a simple rewording: Hawaiian-native culture → native Hawaiian culture.