Expressed as a number, age is always written in figures. If the age is used as an adjective or as a substitute for a noun, it should be hyphenated. Don't use apostrophes when describing an age range. A 21-year-old student.

Is 65 year old hyphenated?

When Not to Hyphenate “Year Old”

If the age comes after the noun that it describes, don't hyphenate it. Here's an example. My mother just turned 65 years old.

Is 18 year old hyphenated?

When to Hyphenate Year Old

“Year old” should be hyphenated when it modifies a noun that follows it. That is, when the phrase is describing the age of a person, place, or thing, and it precedes that noun in a sentence, then it should be written as year-old.