[wrs]

That field is opaque, but the entire type isn’t, no matter what you do. E.g.,

    let x = new Age();
    x.notSoOpaque = 42;
    console.log(x.notSoOpaque);

We can all agree to layer conventions on top of the language so we just don’t do stuff that violates the opacity. But the same is true of assembly language.

[hackrmn]

Assigning to `notSoOpaque` (or any other) property on an object in this case doesn't modify its behaviour, because the property isn't structurally part of the interface -- there's no code defined by the creator / owner of the object (e.g. through the class) that uses it. So it doesn't violate the contract. Private fields are inaccessible, everything else is accessible and is thus part of the interface. I am not saying (and never did) this is the same level as Ada, but your example looks contrived to me -- I don't get the relevance.