numbers/numerals

Spell out zero through nine, use numerals for 10 and above. Spell out numbers at the beginning of a sentence, or, if necessary, recast or rewrite the sentence. However, a number that identifies a calendar year may be used at the start of a sentence; e.g., 2010 was a very good year.

Some exceptions in which numerals are always used for zero through nine:

Ages: She has a 2-year-old daughter. Her daughter is 2 years old. They have three sons, 2, 4 and 6.
Times: 2 a.m., not two a.m.
Dimensions: He built a 4-foot-long fence. He is 5 feet 6 inches tall. The storm left 5 inches of snow.
ALSO: No. 3 choice, 5 percent pay raise, a ratio of 2-to-1.

For more, see “numerals” in the AP Stylebook.