Hola Francis

In Spanish we generally use the noun in singular when it is referring to a plural possessor; we interpret this as "the life of each of them", while this doesn't normally happen in English. For example, you may hear in school something like:

"Abran su libro por la página 20."

Open your books on page 20. 

When the teacher is saying this he is talking to all the students, but he uses the sigular "su libro" as in "each of you open your book", because each person has one book. 

You may use the plural too "abran sus libros", but it's a lot more common to use the singular form. 

Saludos

Inma