[benfle]

If "age" is an important property in your system shared among different kinds of entities then you need to have an Interface or a Protocol to retrieve the age of an entity.

The same way you would create a keyword in Clojure to represent the age of an entity (e.g. ':entity/age') that can be put in a map describing a person or a cat.

In both cases you minimized the interface between your modules and you have less coupling.