Learn Spanish (Spain): Ellas son niñas jóvenes y fuertes.

Ellas is the feminine word for they. Because the sentence is specifically about a group of girls (niñas), we must use the feminine pronoun. If there were even one boy in the group, we would use the masculine Ellos.

Could we leave out the word Ellas and just say Son niñas jóvenes y fuertes?

Yes! In Spanish, the verb son already tells us the subject is they. It is very common to drop the pronoun completely. Using Ellas here just adds a little extra emphasis or clarifies exactly who we are talking about.

Why is the verb form son used instead of somos or sois?

The verb must match the subject. son is the form of the verb ser (to be) that goes with ellos and ellas (they). You would use somos if the sentence was about us (nosotros) and sois if it was about you all (vosotros).

The sentence is about girls (niñas). Why don't the adjectives end in -as, like jóvenas or fuertas?