ages: hyphenation and numerals
How to write a person’s age in running text.
Use figures for a specific age: 7 years old, a 7-year-old. Hyphenate the compound when it comes before a noun: a 7-year-old child, a 40-year-old policy. Do not hyphenate the predicate form: The child is 7 years old. For a decade of age, write in her thirties or in her 30s. Do not write in her 30’s.
Examples
Use They adopted a 3-year-old.
Use Their daughter is 3 years old.
Avoid Their daughter is 3-years-old.
Use He is in his 40s.
Avoid He is in his 40’s.
Spell out an age that opens a sentence, or recast: Seven-year-olds learn fast or Children who are 7 learn fast.