"He writes to the girls and to the boys."
Translation:Il écrit aux filles et aux garçons.
I have to agree with those who think the French translation should have included "les" before "filles" and "garçons". Doing a reverse translation from "Il écrit aux filles et aux garçons" back to English would give you "He writes to girls and boys". It could describe a writer's target audience: "J. K. K. Rowling écrit aux filles et aux garçons, tout de même."
With this sentence, you are learning that "aux" is the contraction of "à" + "les", and therefore "les" is included in "aux".
You have to remember that the French definite articles are used both for specificity and generality.
Therefore, this sentence has "the girls and the boys" as specific and gets "aux filles et aux garçons" in translation.
If the sentence had "girls and boys" as a generality, the French sentence would still be "aux filles et aux garçons" as a generality.
Talking about a writer whose target is "girls and boys", we would say "elle écrit pour les filles et les garçons".
"Aux" is the contraction of the preposition "à" and the plural definite article "les". You use it when the verb is constructed with "à" and the object needs a definite article (generality or specific object).
To write to sb = écrire à quelqu'un
He writes to the girls = Il écrit [à+les] aux filles.
Yes, times have changed and the ways of referring to people and things as well.
However, "une petite fille" (from @3 to @12 years old) and "une jeune fille" (from teens to unmarried young adults) are still in use by those who still care.
The same applies to "une jeune femme" (vs une femme) and "une vieille dame" or "une dame âgée" (vs une vieille femme).