class Person {
      def age;
   }

There is a second-class "age" property: the only way to specify it or use it is to first declare the ADT/Person class. (In this example, I use a dynamic language; that is, the property can be given any type of value.)

Here is the usage of a first-class age property:

   public boolean isOld(Map<String, Object> entity) {
      return entity.getAs("age", int.class) > 30;
   }

Note that in this example, age's semantics are independent of any Person/ADT. Now a statically-typed language that would let me define a vocabulary that include "age" and its type independent of an ADT would be supporting first-class properties.

I could cheat and do this in a typed language like Java like so:

   class Age { int val; } // using class to define a property
   class Entity { Age age; ...and list all other properties here... }

But this is absurdly cumbersome and obviously Java isn't conducive to this.