Hola Laurie

In Spanish, when we talk about possessions in plural, like in this sentence, we can either use the singular or the plural form. So, you could say:

"Ellos han cometido muchos errores en su vida" or "Ellos han cometido muchos errores en sus vidas."

This is something that you cannot do in English, but you can do in Spanish.

Here is another sentence:

"Niños, abrid vuestro libro por la página 8." or

"Niños, abrid vuestros libros por la página 8."

(Children, open your books on page 8.)

I hope this helps.

Thanks for your question.

Saludos

Inma