According to the rules, collective numerals (двое, трое, …, семеро,…) can be used with masculine animate nouns only.
So семеро юношей is OK, but семеро девушек is incorrect, семь девушек should be used instead.
Семь can be used in both cases of course.

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(Personally I don’t “feel” this rule. I read about it in a school textbook and was surprised. I can say «семеро девушек» in colloquial speech and feel nothing wrong. I believe I’m not unique.)