"Les femmes sont des filles."
You can say that they are one in the same due to sex, and calling a woman a girl in English isn't wrong, more so "less appropriate" socially. In fact I'd say that young adults refer to the opposite sex as "boys" or "girls" despite technically and legally being "men" or "women".
I agree that people over the age of 18 are often called girls or boys, but a more correct sentence would be "the females are girls". It may be that there is no age-neutral gender marker in French, but in conversational English you wouldn't generally mix terms in a single sentence as it is confusing.
Of course this could be a limitation of the simple sentences constructed by Duolingo, I just wanted to point out that I found it a little confusing as the sentence translation doesn't mean anything in the same way "the boy drinks water" does.
It doesn't matter if this doesn't make sense in English, it does in French which is the language you are learning now. In any case think about this as something that you just have to get wrong the first time you try it. Languages are different like this as each portray reality in a different way.