When an age is used as an adjective that comes before a noun and modifies that noun, or if the age is a noun, it should be hyphenated. For instance:
- Sixteen-year-old Jayme is excited to get her license.
- The happy sixteen-year-old can't wait to legally drive a car.
- Jayme is a sixteen-year-old. (A hyphen is used because "girl" is implied.)
However, if the age is just a part of the adjective phrase following a noun, a hyphen isn't needed. Such as:
- Jayme is sixteen years old.