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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