NGS Style Manual‎ > ‎- A -‎ > ‎

AGES OF PERSONS, ANIMALS, AND THINGS

Spell out ages of persons, animals, and things from one through nine. Use figures for all numbers larger than nine and all fractions.

Spell out such expressions as a hundred, a thousand, a million, a billion:
  a six-month-old child
He looked sixtyish.
He was six months old.
a three-year-old; age three
flowers nine days old
nine-day-old flowers
at 65 (preferable to at age 65)

in his 30s
twentysomething, also 20-something
fortyish, also 40ish
3½-year-old goldfish
11-year-old structure
50-year-old boat
101-year-old building
the big 6-0

Use age and not aged in parenthetical elements and in compound modifiers:
 
Julia, age seven
five children ages five through 12

school-age children
teenage woes
retirement-age audience
but middle-aged men