Spell out numbers less than 10 in most references. Exceptions include ages, speeds, temperatures, rank, ratios, distances, dimensions.
If a number starts a sentence, it must be spelled out: Twenty years ago, he could ride a skateboard without breaking any bones. He hasn’t been on a skateboard in 25 years. Exception: years and headlines. 2020 was a difficult year for many. 75 people attend justice march (headline example)
Ages: She is a 5-year-old; she is 5 years old. The law is 10 years old. The 2-year-old boy looks like his mother.
Centuries: Spell out numbers less than 10: Table manners in the fifth century foreshadowed many of the practices of the 21st century. Fifth-century table manners foreshadowed 21st-century practices.
