> 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.

What you're referring to is known as row polymorphism or structural typing and it exists in languages such as Elm, Purescript, Scala and Go (and I believe Haskell has these as an extension). For instance, this is valid Scala:

    def isOld(thing: {def age: Int}): Boolean =
      thing.age > 30

    case class Person(name: String, age: Int)
    case class Dog(age: Int)
    println(isOld(Person("Jim", 25))
    println(isOld(Dog(31))

And here's Elm:

    isOld : {age: Int | a} -> Bool
    isOld thing = thing.age > 30

    Debug.log (isOld {name: "Jim", age: 42})
    Debug.log (isOld {age: 10})