"Girls are women."
You would say "The girl is a woman." or "La fille est une femme"
"des" is the plural indefinite article in french (we say "a/an" for singular indefinite, but drop the article for plural)
Hence, "The girls are women" -> "Les filles sont des femmes"
In other words, the sentiment trying to express here is not to say the girls are some particular women ("les femmes"), but simply that they are, in fact, women.
This reply is confusing. In all other cases when the plural of woman is used it is les, this reply does not make it clear why it is changed in this instance. I agree with learn007. It does not make sense. I apologize if I am not understanding your reply. Perhaps it needs a little deeper explanation. Sorry.
On this page, you should read all the other explanations given. Namely, in terms of the structure of nouns in French: <All nouns have to be 'determined' with an article (un, une, le la) or a possessive adjective (mon, <ma, etc), or a demonstrative adjective (ce, cet, cette...), or a number (one, two) or an adverbial <locution (peu de, de nombreux...). To sum it up, it is extremely rare that nouns are standing <alone in French.
In addition, when it comes to the very sense of any sentence or phrase, the choice of the article needed corresponds to a number of rules. One of the rules is that the expression of generalities need a definite article in French (le/la/les) where you have none in English. To mean "women" in general, the French say : "les femmes"
Now, to understand why "girls" translates in "des filles", this is a tip: put the sentence in the singular form: "the woman is a girl" = "la femme est une fille". Then, the plural of "une" is "des".
You need to understand the construction of the sentence as well: the verb "are/sont" gives a definitaion of what the subject is. ("I am a man, we are men..."); then you will understand how the whole thing works together.
Your job here is not to translate word to word, but to understand the structure and what is meant in the sentence to find the relevant translation:
"les filles" = girls in general - in French, always with a definite article (men are stronger than women = les hommes sont plus forts que les femmes)
"des femmes" = "des" here is just the plural of "une". If you put the sentence in singular, you get: "... is a girl. In English, "a/an" has no plural form but in French, the plural of "un/une" is "des".
All nouns have to be 'determined' with an article (un, une, le la) or a possessive adjective (mon, ma, etc), or a demonstrative adjective (ce, cet, cette...), or a number (one, two) or an adverbial locution (peu de, de nombreux...). To sum it up, it is extremely rare that nouns are standing alone in French.