According to the Associated Press Stylebook, spell out all number under ten including zero, any number that begins a sentence should also be in written form, with the exception of the year. Other exceptions to these rules include ages, addresses, times and dimensions depending on the specific style guide.
Always use figures for ages and insert apostrophes when using as an adjective or in place of a noun. Also use figures for dates, years and addresses. Additionally, use numeric symbols for animals, events or things. Use numbers for technical numbers including money, measurements, dimensions, time and percentages. Sizes, height, weight, distances, speeds, temperatures and proportions also use figures. Spell out inches, feet, miles and percent.
When spelling out numbers over twenty, use hyphens. Use symbols for fractions unless they begin a sentence and then use a hyphen to express the division. When spelling out numbers over three digits, omit the word "and" between words. Do not use commas for written numbers above 999. Use roman numerals for wars and to show a succession of people. When spelling out decades, use the lower case; when referencing a period of time using the full year, don’t add an apostrophe before the “s."