Always use digits, followed by the time span. If no time span is given, the number is assumed to be in years: it lasted 17 months; a 40-year-old law; She is 9.

Use hyphens when used as an adjective preceding a noun; otherwise, no hyphens are necessary: the 36-year-old man; the woman is 36 years old.

Use hyphens when used as a noun: acting like 5-year-olds

Do not use an apostrophe when referring to age ranges: in their 40s