TypeScript is intentionally _structurally_ typed (except for enums). The example you describe would only be caught by a _nominally_ typed language (think Java).

Structural typing is great when programming functionally (i.e. with immutability) when the most important thing is the shape of inputs and outputs of functions, instead of named objects (like Person) and properties. In a functional program, for example, "printAge" would likely be called something like "print" or "printNumber" since that is what it is doing to its input _value_.

I think a lot of the misunderstanding I've seen recently around TypeScript (like from the Rails creator) comes from the misuse of TypeScript - if you use TypeScript in an object-oriented way, its going to be significantly less helpful.