Translation:The participants of the competition are young.
The word taught being "participant(e)(s)", the preferred translation must have the English translation for that word, so that when you get it to translate it back to French, you can have a hint of the word you should use.
For your information, "contestants" can back translate to "concurrent(e)s" (usually in sports) or "compétiteurs/compétitrices" (sports or other types of contests).
In English, one would normally say "The participants in the competition. . . ."