In written English, which way is a better or correct way to say? Or any difference?
- Participants are less than 40-year-old.
- Participants are under 40 years of age.
- Participants are younger than 40-year-old.
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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."