"youngs" is not a solution in English. "Young" is already plural and is synonymous with "youth."
It marked, "The young ones" as incorrect and said it should be either "The young men" or "The youth." Isn't it usually possible to generalize the masculine form of a word into either the explicit male form or the more general form, referring to a group consisting of either males and/or females?
You are correct. I like "young 'uns" and "Youth" can be a mixed group, gender wise. Learning to accept Duo's quirks is part of what one signs up for here. ;)