The specific examples you chose to illustrate this point are interesting, because to me, it is clear that what you want is a `Name` type that always represents

The specific examples you chose to illustrate this point are interesting, because to me, it is clear that what you want is a `Name` type that always represents a valid name and an `Age` type that always represents a valid age. Then you can put the validation in a single place (the constructors of those types) and write many methods like `birthdayGreeting` that can happily use values of those types without needing to take on the responsibility of validating them. I honestly don't know of a good way to implement this pattern without type checking or at least optional type hints and static analysis. Instead, you can do validation of the input values in every single method, which is hugely burdensome, or you can assume callers are passing you valid values and write tests to make sure it isn't too catastrophic if they don't. I don't find either of these solutions satisfying.