When you have a true Value Object in your model, whether you realize it or not, it is not a thing in your domain. Instead, it is actually a concept that measures, quantifies, or otherwise describes a thing in the domain. A person has an age. The age is not really a thing but measures or quantifies the number of years the person (thing) has lived. A person has a name. The name is not a thing but describes what the person (thing) is called.
This is closely related to the Conceptual Whole characteristic.