This question is based on the following line in the Wikipedia article on Éponine (a character from Les Miserables):
Éponine is also featured in the stage musical adaptation. She is played by two actors, a young girl for Young Éponine in the Montfermeil scenes, and by a young woman for the adolescent Éponine in the later Parisian scenes. Actresses playing Young Éponine and Young Cosette sometimes interchange roles in different performances to equalize performance opportunities and vocal strain.
Is this something only young actresses do, and not older ones? If so, why - is there a biological explanation, or something else? The sentence is not cited, so it could also be wrong, in which case that would also be an answer.