Use when deemed relevant to the situation. If someone is quoted as saying, “I’m too old to get another job,” the age is relevant.

Generally, use ages for profiles, obituaries, significant career milestones and achievements unusual for the age.

Use ages for people commenting or providing information only if their age is relevant to their comments (e.g., a teenager’s comment on video games aimed at that age group).

Appropriate background, such as a parent of two young children or a World War II veteran, may suffice instead of the actual age.

Always use figures. 

The girl is 15 years old.

The law is 8 years old.

The 101-year-old house.

When the context does not require years or years old, the figure is presumed to be years.

Use hyphens for ages expressed as adjectives before a noun or as substitutes for a noun.

Examples: 

A 5-year-old boy, but the boy is 5 years old.

The boy, 7, has a sister, 10.

The woman, 26, has a daughter 2 months old.

The race is for 3-year-olds. The woman is in her 30s (no apostrophe).