In written English, which way is a better or correct way to say? Or any difference?

  1. Participants are less than 40-year-old.
  2. Participants are under 40 years of age.
  3. Participants are younger than 40-year-old.

"40-year-old" is often used as a noun, as in "a 40-year-old", so the first and last are awkward. The second is correct and could also be

Participants are under 40 years old.

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  • Also, if it's obvious we are talking about age, that version can be shortened even further: Participants are under 40. A couple examples: Shivery .. estimates about seventy-five percent of their customers are under 40. Also: In China, the average age of new-car buyers hovers around 34. Thirty-eight percent of all new luxury car buyers there are under 40. – J.R. Aug 9 '17 at 20:24

Participants are less than 40. Participants are younger than 40.

This would probably be how I would say it.

Both of "less than 40-year-old" and "younger than 40-year-old" are incorrect. If you want to retain the "years old" concept, you would just say "less/younger than 40". "Participants are under 40 years of age." is correct, but clunky - I would say "Participants are under 40 years old." instead.

Dashes ("40-year-old") are generally only used as a "compound" noun when identifying a person - "I have two children, a 3-year-old and a 5-year-old."

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