• Always use figures. If ages are expressed as adjectives before a noun or as substitutes for a noun, use hyphens.

The child is 2 years old.
He just turned 15.
It’s a 3-year-old program.
The room was full of 7-year-olds.

• Use age (not aged).

Morgan, age 4, doesn’t appreciate dry humor.
That video is intended for a retirement-age audience, not a teenage one.