Always use figures. When the context does not require years or years old, the figure
is presumed to be years. Ages expressed as adjectives before a noun or as substitutes for a noun use hyphens.
ie. A 5-year-old boy, but the boy is 5 years old. The woman, 26, has a daughter 2 months old. The law is 8 years old. The race is for 3-year-olds. The woman is in her 30s (no apostrophe).