That's for English. How about for Danish? Can we also put "et hundred år gammelt" or does it has to be without "et".
It seems to me that I have seen sentences with "et hundred" and others with simply "hundred". So I am a bit confused.
Certainly in British English, "stamp" with no other qualifiers would normally refer to a postage stamp. Unless the context made it obviously something else (eg a teacher talking about using a stamp for marking schoolwork).
A century is a hundred years long, but I don't think many English speakers would use those terms interchangeably. "The stamp is a century old" sounds odd to my ears; although I would obviously understand what you mean.