adjective
1
BAD : He is married to a twenty years old American girl.
GOOD : He is married to a twenty-year-old American girl.
BAD : Eight-years-old Sarah had a few surprises up her sleeve.
GOOD : Eight-year-old Sarah had a few surprises up her sleeve.
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Use years old after the verb be : 'Sarah is eight years old.'
In front of a noun, use a compound adjective (WITH two hyphens and a singular noun): 'a three-week-old baby', 'a ten-year-old daughter'
Nouns of measurement ('year', 'week', 'gram', 'mile' etc) are always singular when used in compound adjectives: 'a ten-second silence', 'a six-minute wait', 'a five-mile race'.
2
BAD : He fell in love with a young girl of nineteen years old.
GOOD : He fell in love with a young girl of nineteen.
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noun + of + number: 'a child of five', 'a man of sixty'
Compare: 'The girl was nineteen years old.'
3
BAD : The old should not be brushed aside by society.
GOOD : The elderly should not be brushed aside by society.
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To refer to old people in general, use the elderly : 'The building has been converted into a retirement home for the elderly.'