Could it be that you and Anonymous Monk made the same mistake as I did initially of reading the example as if age() was meant as a read-only function?
As I understand the example, age() is a write-only(!) function, you set the time you want to stow away the cheese in a dark cellar (or you simply stow away the cheese a predetermined time). The following sentence in the book makes that quite clear I think. So 'ascertain' would be exactly the wrong meaning, suggesting reading of a stored value.
If I'm right, you made my point ;-). And in any case thanks for the compliment.
PS: If I'm right, the second age() call in the example following the cheese paragraph should have a parameter of "6", "6m" or "6 months"