Generally, spell out whole numbers one through nine, use figures for 10 and above. Use figures for dimensions and percentages. Grade levels are an exception—always spell them out. Check the Associated Press Stylebook for exceptions. Spell out numbers at the beginning of sentences. Use figures for ages including 1-9. Avoid using fractions of years; use months.
Right: 16 buildings
Right: 4 inches
Right: He teaches eleventh grade.
Right: She has a daughter, Jill, 3.
Right: His son is 18 months old.
Right: Twenty students registered for the class.
Right: 8 megabytes, a 2-gigabyte hard disk